If the price is the same or close, I would get the Athlon 64 system. For office and internet surfing, you won't notice a difference between them. The Athlon 64 chips run 32 bit programs just as well as, if not better than, the 32 bit Athlon chips. For video and media encoding the Pentiums are slightly faster but again, unless your into serious editing, you won't notice too big a difference. The Athlon 64 will give you a little more room to grow in the future if you plan on keeping the computer for a couple of years.
The big issue with 64 bit chips is that for most people the only apparent advantage is that you can use far more RAM than the 4GB limit for 32 bit processors. Most people don't need anything close to this right now. There will be advantages though with games and graphics programs when they start optimizing them for the 64 bit chips. There are 64 bit versions of Far Cry and UT2004 in development, and probably Doom 3 too.
goates