Get an Abit KT7A mobo (runs at 133 fsb, much better for future upgrades and faster now), and a T-Bird processor. The 1.0 ghz are at a sweat price and a magic number. Then learn how to unlock the multiplier and you can probably get some more speed out of the chip for free by some light overclocking. (Do a search on <pencil> on the forums, and in Anand's articles, to learn how.)
T-Birds are a much better deal that coppermines, not to mention P-4's. Also, PIII coppermines are at the end of their speed ram now. No upgrade path if you buy a PIII motherboard. P4's are due to change motherboard (more pins on new P4's) in the 2nd half of 2001. So you gotta chuck that mobo too if you want to buy a new P4 in 9 monts or a year.
Not so the KT7a. You can plug a Palomino upgrade cpu in that board in 9 months to a year for probably only about a $150 upgrade. Sell your old 1.0g @1.2g T-Bird for say $75 then, for a net upgrade price of $75 plus shipping. Very realistic. Leaves room in the budget to buy that NV20 video card when it comes down in price and games start being released that really use it.
If you want to go a little cheaper now, just get a little slower T-Bird. Stay with the KT7a regardless though. For your next upgrade.