Why choose XP or 7, take both! At least that's how I'm set up, XP and 7 in dual-boot. I only use that setup as protection, if something goes horrendously wrong with Win7 or the HDD it's on, and it's a problem that might require format and reinstall, I can always boot in XP and deal with the problem pretty easily.
System requirements for 32-bit Win7 need only 1gig of ram, whereas the 64-bit needs 2gigs. The comment about getting 4gigs or more only really applies to the 64-bit variety since it's ram hungry. If I'm not mistaken the 64-bit version, if you have 4+ gigs, will allocate a portion to the OS and nothing else. I've got 5gigs total, but only 4.5gigs available for use. It's done to reduce conflicts for ram between the OS and other programs. With no programs running, Win7 Ultimate, and all features enabled (such as Aero styles, etc..) my ram use is 30%. If I disabled all the resource heavy features I could probably cut that to 20% or so.
Personally I always loved Windows XP, and it took me awhile to decide to switch. But in all reality Windows 7 has better features, better functionality, is very very easy to use, and there's also the issue of OS updates and support. I'm not sure if XP is still supported, but even if it is it won't be forever.
In short, Win7 is filled with awesome. But the 64-bit needs a solid system to function. The 32-bit is much less demanding, but the cost is that it's also somewhat less capable.