Okay, somebody had to mention AMD, so I guess it will be me.
Why did you rule AMD out ?
Anyway, as many others have mentioned, picking an intel platform isnt too easy these days. The P3 is dead-end since Tualatin will be comming out, and requires a new motherboard. Its your money, but I would never buy a motherboard that couldnt handle at least one or two cpu upgrades. P4 is also dead-end, as you know Northwood is comming out. However, if you have the money for a 1.7, it should last a while. Throwing a motherboard away after 3 years doesnt hurt that bad.
Also concider the memory issue. If you go for a P3 now, you may want to buy a DDR board, and get DDR ram. Not that it helps performance, but that at least then you dont have to throw away mb+ram+cpu in year or so. If you go the P4 way, you'll end up with RDRAM.. not too sure how long even intel will support this memory type, but its probably safe to say you might upgrade to Northwood later, and keep your RDRAM (is the 1+Ghz RDRAM already out ?).
From a performance point of view, a P3 1Ghz would be sufficient now, and next year, and maybe even longer when combined with a high-end videocard for gaming. But still, you'll have to upgrade it one day; so keep that in mind.
If I were you, I'd buy an Athlon system. Something like a 1 Ghz Tbird would suite your gaming needs perfectly, and you can always upgrade it a faster cpu (maybe as much a 2+ Ghz Barton one day). If VIA scares you, get a SiS or ALI based motherboard.. or an AMD 760 board.
If you insist on buying intel, I'd wait for Tualatin. If you read toms preview, you'll have noticed it performs quite well, and at least you'll have a moderate upgrade path (dont know how fast they will make them.. 1.2 at least, but maybe more.. 1.4+ should be achieveable I guess).
An other option would be to wait for northwood. Though it will most likely be overkill for todays games, this might be a system you'll hang on to for many years, and be able to upgrade.
In short, any platform you buy today (heck, maybe even a 850 Celeron) would be fast enough for todays games when combined with an apropriate videocard. P3 and P4 are dead end streets, but if that doesnt bother you, you cant go too wrong with either one for gaming.
If you have to buy today, a tbird would make most sense to me. If you can wait, I think a tualitin would be a good alternative. If you got money to burn, a Nortwood system might be an excellent and lasting choice.
Just my 2 cents though..
---- Owner of the only Dell computer with a AMD chip