I agree with SOB, go for PIII 800 or similar, first of all its relatively cheap to obtain, secondly its highly overclockable.
With some preparations (mainly by getting information about overclocking before doing it) you can get almost 20% performance increase from the PIII800. Now that makes a really good price/value..
I myself am running Aces High with Celeron667 overclocked to 950Mhz and its running solid, fps ranging around 60fps at 1024x768 32Bit color. I have Geforce SDR 256 (overclocked of course.)
The most important thing to consider when upgrading is that you get a system which is good in all areas..
First of all you'll want upgradeability so when you choose your motherboard, go for the big names on the market like Asus or Abit (I suggest reading articles from places like
www.ocshoot.com for further information..)
You'll need enough system memory, at least 128Mb but 256 is advisable these days, make sure its PC133 minimum (that way, if you get PIII with 100Mhz Front bus speed you'll get 33% in-spec room to play with your memory for overclocking your chip.)
After you've setup your mainboard and processor, you have to consider upgrading your display card. Geforce 2 is the only way to go if you want to get the best (albeit not ultimate) performance from your new PIII system. If price is an issue, Geforce 2 MX is a very good price/quality item - however when upgrading like this I strongly suggest you take the Geforce 2 since the MX model will not be able to make use of your new cpu processing power.
Finally, the monitor: Anything from 19" up is good. Some people say that 21" is too big if you sit up close to the monitor - thats a matter of opinnion. I'd say you'll be fine with 19" and AH. Just keep in mind that you NEED to CHECK the monitor BEFORE buying it.
Monitors (same brand, same model) have huge differences amongst them. They never leave the factory with exactly same settings so you may get a 19" trinitron screen with a foggy and blurry picture even if the same model at shop seemed to be crisp clear and sharp. Trinitron and Diamontron screens are best overall quality, but they do have some issues with Geforce line videocards - it is imperative that you test your monitor first with a geforce videocard before buying it. Otherwise you may end up returning it for another one anyway.
I hope this advice will help you with choosing your next system setup.
- MrSiD