If you have 4 gb-ram and a graphics card with dedicated memory it is a good idea to upgrade to a 64 bit os, just because it supports more ram. 32 bit only supports a total of 4 gb-ram seeing the graphics card ram and normal ram as the same memory, so upgrading also gives a little boost to your graphics card ram.
This is my graphics card memorÿ from a dxdiag file:
Display Memory: 2280 MB
Dedicated Memory: 506 MB
Shared Memory: 1774 MB
And for sure I can see the advantages of 64 bit, I can play nfs shift on my laptop on high graphics, and a guy from school with the same laptop can play nfs shift only on medium/low graphics on a 32 bit system.