About to purchase a new pc and I know next to nothing about processors and which one to get. The three options are:

Intel® Centrino® Duo Processor Technology - Intel® Core™ 2 Duo Processor T5450 (1.66GHz/ 667 FSB/ 2MB Cache) with 2GB (2x1GB) Dual-channel Memory 667MHz DDR2 SDRAM Shared With Graphics

AMD Athlon 64 X2 Dual Core 4400+ (2.3GHz, 512KB + 512KB) with 2GB(2x1GB) DDR2 SDRAM at 667MHz

Intel® Core™2 Duo Processor E6420 (2.13GHz, 4MB L2 Cache, 1066MHz FSB) with 2GB(2x1GB) DDR2 SDRAM at 667MHz

Assuming that i'd want to use the pc for three years for general purposes and possibly gaming, which one should i get? Please bare in mind the third option is around AU$400 more expensive and for around AU$200 i can double the RAM on any option. 

From a gaming perspective, most games are far more GPU limited than CPU limited so I don't think you'd have too much trouble with any of these processors ...

Depending on what else you intend to do would largely determine which processor you buy; if you're doing a lot of video conversion (from DivX to DVD) many of these programs are single threaded so it would be a good idea to look at which is better at single threaded applications, while 3D rendering packages tend to be multi-threaded.

If you're mostly using it to read email, work in MS Office (and what not) I would go with whichever is cheapest.

AMD > INTEL, ALWAYS

i got a 6100+ (overclocked) and im very happy with it

Right now AMD is probably the best value for money. However, you will get a better performance out of the Intels. In reality, it's just a matter of how much you want to spend versus how much you want to game =) You'll find that the Core 2 Duo will overclock really well, so you might want to go with the Core 2. It also uses less power, which means less heat and less noise.
As for gaming, your choice of graphics card is important but don't discount the necessity of having a good CPU. I think the cheaper CPUs would both struggle to last a long time

The E6420 would be a lot faster than the other two so I'd go with that over an extra 2 gigabytes of ram. One of the reasons is because most motherboards that support the E6420 also support intel quad core processors so it would be easier to upgrade in a few years rather than having to buy another motherboard and cpu. And as ram prices drop (well actually they'll be going up soon) but in a year or two you'll be able to pick up 2gigs of ram very cheap.

I think there's supposed to be a price drop for the Core 2 Duo processors this month though (at least in North America but I assume it's a worldwide price drop). And there has already been a price drop on the 4400+ (at least in Canada so I assume everwhere else) it dropped from $141.45 Canadian to $99.99 Canadian (average retail price)  a few days ago.

Get this one man.

http://www.newegg.com/Product/Product.aspx?Item=N82E16819115017

Anyway, it all depends on how much you want to spend

Here are some benchmarks comparing the processors:

http://www.xbitlabs.com/articles/cpu/display/core2duo-e6420_6.html

E6420 every time.

the T is a laptop cpu, don't get.

Really helpful guys, thanks for taking the time