I don't want to spend more than that.

I want to use the PC mainly for gaming... and I want it to at least have a GTS 250 or a 9800GT as a video card. Is there a cheaper ATI solution that provides the same (or better) performance as the GTS 250?

If possible, I also want to use it to emulate PS2 games... so I guess I'll have to go with an Intel processor. I've heard that Dual cores perform better than Quad cores with PCSX2 (PS2 emulator), is that true? I want to run the emulator without framerate drops or low framerate :S Or maybe I should forget about emulating PS2 games and just get an AMD processor which will make the price lower?

I also want at least 2GB of RAM... but I don't know if I'll have to get DDR2 instead of DDR3 to lower the cost (if it's not enough)?

The prices in my country are higher than in the US... so, you can search for the PC parts here to know how much they'll cost me:

www.mercadolibre.com.ar

Buscar = Search

Nuevo = New

Usado = Used

I don't want anything used :P

So... am I asking too much for just 620 dollars or should I be OK?

Oh, and BTW: 1 Dollar = 4,01 Pesos

You're probably asking too much for $620USD, thanks to where you live :/

2GB of RAM just doesn't cut it these days. You need at least 4. And at the most 8. I would recommend getting a pre-built computer and just buying a graphics card.

So it would be enough if I was in the US? that sucks xD

But pre-built computers usually come with a crappy motherboard which I'll probably have to change when I want to buy a video card and they also come with a very limited power supply... the case will probably have to be changed as well. Not to mention that pre-built computers usually cost a lot more than they would've cost if I bought all the parts separately :S

Should get an 

LGA 1156 Motheboard 

Core i5 processor

4gb DDR3 ram

HD Radeon 3000 series will do 

In the us that would cost around 450-500 dollars roughly. Add some postage and you could get it to you for underbudget. 

Also dont buy any of those combo's and look overseas like the US. And remember to check if they post to you and there postage costs. And Look around. 

Uhh... what kind of performance could I expect from an HD Radeon 3xxx? If I end up with something simillar to what consoles have to offer then I'd rather wait until prices go lower and buy something better in the future and keep playing with my PS3 in the meantime.

I wanted to buy the parts from that site because most of the sellers there "skip" customs so they don't have to pay any taxes and their prices are a lot lower than those who do everything legally and pay taxes at customs. Let's say I buy what you told me and have it shipped to my country. The people at customs will most probably "freeze" my computer parts at customs and they'll give me a call to present myself there. When I go there, they'll add 50% of the cost of what I bought (500 plus 250 = 750) in taxes and 21% more in... more taxes, lol. It'll end up at almost 1000 dollars...

But in this "black market" if you want to call it that way, they'll just Illegaly get the parts into my country (avoiding taxes) and they'll just add a little bit more to the price (nowhere near to what I'd pay if I did it the legal way, lol).

So I have no choice, lol.

By the way, look at this and tell me what you think:

http://articulo.mercadolibre.com.ar/MLA-103596596-7013-cpu-pc-amd-athlon-quad-core-4gb-ddr3-1333mhz-dvd-_JM

Motherboard Asus M4n68t-m le

AMD Athlon 2 X4 635 2.9GHZ

4GB DDR3 Superlatent 1333mhz

HDD 320GB SATA 7200rpm

DVD burner 22x Dual Layer

Asus GF 7025 256mb Onboard

6 USB ports 2.0

Full case 450W power supply plus 1 lateral vent

Multimedia keyboard, optical mouse and powered speakers

Total: 1530 Pesos (381,55 US dollars)

I could maybe add a better video card and a 600W power supply later... what do you say?

Here's another one with an i5 760:

http://articulo.mercadolibre.com.ar/MLA-104566314-7130-pc-quad-core-i5-750-dvd-2gb-ati-4850-1g-hd-500-_JM

Motherboard INTEL Dp55wb LGA1156

Intel Core i5 760 2.80ghz

2GB DDR3 1333mhz

HDD 500GB SATA 7200rpm

DVD burner 22x Dual Layer

ATI Radeon HD4550 1GB

8 USB ports 2.0

SENTEY case with 600w Power supply

Total: 2710 Pesos (675,82 US Dollars)

This one is pretty awesome... it costs more than 620 dollars though but... I have the money :P I could spend a little bit more to get this one which should be awesome for emulating PS2 games and it is also good for PC gaming and I won't need to go and buy another power supply. I'd still need to replace the HD4550 for something else though... which could cost quite a bit more.

What do you think would be a better idea? getting the first (cheaper) PC and get a 600w power supply and a better video card (I don't need a better motherboard too to avoid bottlenecks, right)? or the second (more expensive) one and replace the HD4550 for something else?

Given your circumstances, I think a prebuilt will be cheaper as you say (those video card prices are insane).

So the first computer you mentioned with a GTS 250 or HD 4850 or HD 5750 or GTS 450, whichever of those is cheapest. You don't need to replace the power supply; 450W is enough. The rest of the components are fine.

I don't think you need an Intel processor for PS2 emulation? AMD is compatible with all x86 instructions except SSE4.x as far as I know.

my bad with the graphics card, i didn't know which one to pick so i went with an low one. 

you can get an 6850 for 250 USD  

anyways you have an 450W power supply for that Pc. I dont believe that would be enough. And with power its better to have more than less because less can cause alot of problems.

That would barely be enough for a decent gaming PC even in the US.  You could build one but it would be bottom of the line and would run most current games at lowest settings.

And then of course next you probably couldn't run anything new.

-edit-

Also, don't forget to factor in $100-200 for software and accessories.

I doubt a quad-core with 6850 would consume more than 250W total power. TDP ratings are very conservative.