For the uses you have specified, you do not need core duo chips. Are you going to need Vista? Do you want to play games (if so, buy a bloody games console!).
For straight 'office' type work - surfing, emails etc, even the cheapest, slowest thing Dell sell will be more than adequate. For XP don't go below 512Mb RAM. Just had to spec up a couple of machines for my brother to use in his office - 2.8G P4 dual core, 1Gb RAM, 250G HDD, 17" lcd, DVD writers, wireless keyboards/mice comes out at about £450 each inc delivery (and a coupon for Vista when out, not that he'll use that), though you can get cheaper.
Celeron is good budget chip if you only need to do office type work.
P4 is particularly good at multimedia and video editing. Fan noise in many systems could become a problem due to cooling requirements (Not Dell, as they use the excellent BTX system in most systems)
AMD chips (all variants) are superb at integer type work, and last generation games. Fan noise as above. Weak in multimedia/video compared to Intel.
Core 2 Duo is the current king in the x86 arena (forgetting about the quad cored extremes).