Here's a page from Anand's test of the 1.7GHz Celeron, which was run on an i845D chipset:
link. The AthlonXP's in that test were running on the KT333 platform. The 1.40GHz AthlonXP is definitely outperforming the 1.70GHz Celeron there. Considering that an AthlonXP 1700+ now costs $49 (versus $53 for a 1.7GHz Celeron or $83 for a 2.0GHz Celeron), it looks like AthlonXP is going to be the better buy in the CPU department.
Motherboards are a separate story, of course. One of the most popular AMD boards is the EPoX 8RDA+, which is about $95. If you were looking for a value platform for the Celeron then probably an i845PE board like the EPoX EP-4PEAD would be a good pick, about $72. Both boards will let you upgrade to a faster CPU down the road.
If you're truly trying to cut costs, and happen to have some decent PC133 you want to re-use, consider the ECS K7S5A as well... it has slots for PC133 as well as DDR, and it's under $60. It will take 266MHz-based AthlonXP's but not the newer 333MHz-based ones (or not at proper speed, anyway). But PC133 will drop the performance up to 10% in some situations, versus PC2100 DDR, so keep that in mind.