I run 64bit vista ultimate on my desktop, and 64bit ubuntu (with vmware xp pro 32bit) on my laptop. 64bit has come quite far in driver support etc so its quite usable. Also the ability to run 32bit programs has become a lot more stable and I have yet to really hit a program that doesn't work. Terragen runs great, and I mostly can't wait til it can use multi-core. Also Terragen is still usable in a 64bit os, it just still has to run with 32bit limitations until the 64 bit edition comes out.
Another quick note: If your planning on 4 gigs of ram, yes 32bit "technically" supports it, but odds are you'll not have the ability to use all of it within windows due to how memory is added n such. Like the graphics memory is loaded first (so i have 1gig of graphics ram, so I was already down to 3 gigs of my 4 gigs of ram, then another chunk goes off too to from something else, so i registered like 2.65 gigs of my 4 gigs of ram).
So, if you want to use all 4 gigs, or are thinking of maybe more (since ram is cheap, i plan on buying another 4gigs later), i highly suggest 64bit.