What about something like a Pentium 4, a Northwood on socket 478 or an Athlon 64 on socket 754? Bet you can pick them up on eBay for less than the price of a takeaway!
You could install Win 98 SE on the machine too and go proper retro!
I built a similar machine for my brother a couple of years ago and he loves it, he finds it easier than using emulators and virtual machines.
You could also just look around for an old, used office PC with an AGP slot and get a suitable graphics card like a Radeon X800 or something, probably want an extra stick of RAM too, again off eBay.
EDIT Though the old, used office PC would probably need a bigger Power Supply if you were going to add a graphics card!