Cant say anything useful about PS since I dont use it - 32 or 64 bit, but Clercon is right. Whilst you would get more system memory and more memory for the whole PC to use, PS on its own running as a 32 bit app would be limited to 4Gb of memory on your squillion Gb shiny 64 bit box. Memory wise, it would be better than PS on a 32 bit box but only by about 1Gb max. You could not use 6 or 8 Gb of ram with it. Generally, 32 bit apps run on 64 bit windows just fine. You only NEED 64 bit windows device drivers for a 64 bit OS and nowadays you can get 64 bit drivers for almost everything - certainly everything sold new.

Most new CPUs now are 64 bit anyway. Most new motherboards have DDR3 and a 4Gb stick of that is about £18 and most motherboards have two slots min so getting 8Gb is no issue now. The OS is either XPx64 (what I use) or Vista64 (just don't do it !) or Win7 64 bit, which is like Vista64 only this time it works. If your buying a new machine and want more than 4Gb PS and are not going to shell out for the upgrade no matter what then consider making a machine up from bits or buying it with no OS and installing Ubuntu x64 or Linux Mint and trying the Gimp in 64 bit or even Xara (free in linux). If you dont get on with it then you can always format the hard drive blank again and buy a windows copy and install that and go again. If going windows then I would definitely recommend getting a boxed copy of the OS even if its the somewhat iffy OEM type version. But don't just get it pre installed and live with it as you would be on a limited life span before you need to reinstall it again and lose your ability to get your work back. Oh and on that point, budget for an external USB hard drive as back up. It amazes me how many posts we have on this forum about lost work. Losing the last thing you were working on is unfortunate, losing everything you have ever done is just careless.