Windows Vista Home Premium x32 or x64?

Ok, I know this question has been asked a million times already but I looked all around and I just want a straight answer.

This site Reader Question: Maximum Memory in 32-bit Windows Vista - Windows Vista help says I should use x32 but a lot of other forums say to use x64 when you have 4GB+

Computer specs:

OS: Windows Vista Home Premium (x64)
Processor: Intel(R) Core(TM)2 Duo CPU T5750 @ 2.00GHz
Hard Drive: 250 GB
RAM: 4GB
Graphics Driver: Mobile Intel 965 Express Chipset Family (Not sure if this is correct, also is it possible to change this to NVIDIA or ATI without having to buy something?)

I use my laptop for playing games (Halo, Gears of War) and watching movies also I use it to play MMO's, so perhaps a better processing and download speed?

If you need any more information please tell me.

Look at it this way. If you want to do a lot of processor intensive stuff and your rig runs 4+gb of ram: 64Bit

If your rig is basic and you just check e mail, browse the internet, basic computer stuff: 32Bit, as it needs less resources.

I want to download a lot of games and be able to play them without lagging

Run 64bit. As you can run more ram with that architecture.