Microsoft has an upgrade option that you can purchase. As far as performance it depends on what software you run. For normal PC use there isn't much advantage to a 64-bit O/S unless the software has been properly written for it, which most common software has not IMO so either O/S will work about the same.
Go a head and install more than 4 GB of ram in your computer. 32 bit OS won't be able to address the extra bytes so you will max out at 4 GB of ram. This issue of performance with 64 bit is really negligible compared to the performance hit of pageing to harddrive and or having a decent CPU in your system.
4GB - (VGA Memory + Other Resource Allocation), which usually comes down to roughly 3GB (provided you have a 1GB card).
Anyway, as Braebrun pointed out unless you have more RAM, you'd be okay with x86 version of windows, also, if you use applications which can address more than 2GB of memory addressing then you'd be much better off with x64 windows.
ASUS P8P67 EVO Motherboard
Intel Core i7-2600K Sandy Bridge 3.4GHz (3.8GHz Turbo Boost)
CORSAIR DOMINATOR 8GB (4 x 2GB) 240-Pin SDRAM DDR3 1600
COOLER MASTER HAF X Blue Edition
ASUS ENGTX580 DCII/2DIS/1536MD5 GeForce GTX 580
2-Western Digital Caviar Black 1TB 7200 RPM 32MB Cache SATA 3.0Gb/s
Crucial RealSSD C300 128GB SATA III MLC Internal Solid State Drive (SSD)
SONY Black 12X Blu-ray
Thermaltake Toughpower 1050W Gold
This is the PC I run and it has Win 7 32 Bit I plan on playing BF3 and BC2 along with the COD Series Would it be significant to install 64 bit? Whey would anyone want 32bit?
The system you list would appear to suggest you wanted a decently fast system for gaming and other applications. 5 gigs of your ram are actually being wasted with the 32-bit Win 7. With that type of setup, you should really run the 64-bit version of the OS. Although there are not a ton of 64-bit apps around, they are there.....and more are coming. A gaming example would be Crysis 2. Let's say you wanted to play the full DX11 ultra version of crysis 2 with the hi-res texture pack installed. The hi-res texture pack is unusable unless you have a 64-bit OS, so even though you put together a powerful system, because of the 32-bit OS ( yes, here comes a play on the old overused saying ) ......it won't play Crysis 2 ( at least with hi-res textures ).