Looking for some help

I built my pc about 4 years ago. Here are the stats.

Motherboard - 680i LT
CPU - Intel Q6600
GPU - GeForce 9800gtx+
Memory - 2 gigs OCZ DDR2 sli + 4 gigs of kingston bargain ram (for no good reason since I am running 32 bit windows 7)

Here are my questions. First off it is important to know that this is a gaming rig. I run my system through my Vizio VGA slot at 1360x768 resolution. Most games at that resolution can be fully blown with next to no issues. Games in question Crysis 2(Max), Dragon Age 2(Max), Mafia 2(Max minus physx option). The first game to really challenge my rig is Witcher 2. My frame rate even on low settings is well below 30fps average. I was thinking about a gpu upgrade but before I toss a couple hundred bucks into the system, I was wondering what I could expect from such an upgrade. I was planning a new system for the early part of 2012. At that time I am going fully blown with an i7 and finally make the move to sli. If anyone has suggestions I would much appreciate it.

Last question. What would the benefit be going with a 64 bit os vs 32 for my use. I wouldnt expect much even with the extra 2 gigs of ram recognized. Still I would rather hear responses. Also a side note I was planning on the 1gig 460 as my upgrade card and tossing the 9800gtx+ as a stand alone physx card.