www.xbitlabs.com
my favorite review site out there for comp parts. they are biased towards AMD and ATI, but justifiably so(best stuff out there)
In my view you have 3 options
1buy the budget processors now that they have listed along with mobos that work with them(amd 2500+ and up).
2buy the top end processors(400 dollars) and mobos that work with them-784 chips if you go 64 bit athlon
3wait a couple months for the 64 bit models to come out, at which point the 3400amd XP should be at a decent price(sub200$)
for athlong the top end are the 64 bits and intel its the P43.4ghz
you can get pretty decent stuff for 350 dollars(including 1 ghz of memory) if you get a new mobo,proc, and ram. If you go this route the top end of the budget line will meet your HTPC needs, although barely. The new software being used is pretty intensive.
The other 2 routes will be better, although they are more expensive. I personally would wait a couple months(also do some research on what you are doing first) for the XP 3200/3400 to drop in price.
Whats your other equipment?