I've been building computers for years. I will tell you this much, that you can not build a computer to rival the PS3 graphically for 500$.
Let's start with some facts.
FACT #1
Just becuase the PS3 has 7900GTX * 1.5 or a 8800GTX * .75. Does not mean that a 8800GTX will look better than what is on the PS3. Why? Because of optimizations, and hardware shortcuts. This type of optimization can not be done on a PC easily. That is why PCs are going to Unifyed Shader Models, as they scale MUCH better. Overall the Graphics chip on a console gets 4x more power than an 'equivilent' graphics card for the PC.
FACT #2
You need at least a dual core processor to even compare to the 360. Screw compareing to the PS3. It's not gonna happen.
So here we are. I will build a computer that compares to a 360, becuase the CPU on the PS3 cant be rivaled with CPUs on the market today.
CASE: 89.99+15.99 = 105.98 (the case needs to be of good design to keep noise, and cool the graphics cards)
Motherboard: 89.99+6.61 = 96.60
Processor: 222.90 (It's not the cheapest processor, but the best bang for buck processor, when you OC it)
PowerSupply: 79.99 (Once agian it's not the cheapest, but it's one of the cheapest that will last/work)
Memory: 118.99+4.99=123.98 (2gb cheap memory)
HDD: 36.99+6.14=43.13 (cheap-ass 80gb HDD)
Graphics Cards: 279.99+6.33=286.32*2=572.64
Windows Vesta: 214.99+4.99=219.98
This dosent include quite a few of the things you need to build a computer, namely a DVD drive, and a few other nit picky things I would point out.
But with what it costs to build the system itself.
$1465.20
And for me, I have to pay 9.25% sales tax on top of that.
$1600.73 is the total for me to build a system that truely rivals the 360/PS3
Now you wanna show me how you build a system that can do what the PS3/360 can for 400-600$ Be my guest.