I'd say get the 64bit, lol for the same price. DO NOT however use Windows XP 64-bit unless you want problems with your current applications. A 32-bit application running on a 64-bit operating system is a no no.
You shouldn't run a 32-bit operating system on a 64-bit machine either. Unless you're going to get the 64-bit OS, stick with getting the 32-bit machine. "Mixing the bits" is a dangerous game, as many compatibility issues can and will arise. Using 64-bit will also require you to purchase more memory to get the same performance that you would get with 32-bit.
As far as Vista vs XP goes, Vista is better after you disable UAC, and if you're concerned about the resource hogging, you can disable Aero. Windows Defender was on XP, and all it does is protect you, the number of interactions that I've had with it on Vista have been minimal, and it uses barely any resources. You also need Vista to run some of the newest games to their fullest potential. (DirectX 10)
A true computer geek wouldn't buy a computer from a manufacturer, but rather order the individual parts from a supplier and assemble it themselves. Until you can do that, don't call yourself a computer geek.