PIII 500mhz or PIII 1ghz+, it makes a big diff.
Anyway GF4MX420 will give near GF2GTS 3D perf but has the GF4 enhancements which are image quality, dual display, TVout and hw DVD playback (MX versions). So it's much like Rad7500 although Rad7500 is a little faster in 3D and tends to be slightly better than the GF4MX in the other depts too. GF4MX440 is available in PCI format as well but is still hard to track down, it's faster than GF2TI and Rad7500. Rad9000 is out outside the US and although it only offers GF4MX440/Rad7500 type 3D speed it is the only PCI card to impliment DX8 hw just like Rad8500/9000 and GF3/GF4TI. Rad9000 should o/c well, 20% should be doable esp with 4.0ns DDR RAM which would make it clearly the fastest PCI card. I don't think Rad9000PRO is scheduled for PCI treatment so Rad9000 is tops and is about £105 in the UK, so it should translate to about $105 US. The perf penalty from switching to PCI from AGP is about 20%, so expect a Rad9000-PCI o/c'ed 20% higher to only give the perf of a standard Rad9000-AGP, but that's about as good as PCI is ever likely to get. Do consider an upgrade as SktA and AthlonXP could cost under $100 and are hugely better. $100 will get you a Rad8500LE-128MB AGP which will cain a GF3TI200, in fact $120 can net you a GF4TI4200 AGP, but then this is the price you pay for needing a PCI gfx card.
RAM is very important in PCI gfx cards, 64MB is the absolute minimum do not go any lower. Also the precise gains vary by game but even more so the CPU. The closer you are to 1ghz the more worth it the faster cards are.
Digit Life - Rad9000 article (also check the site for other great info esp on Rad9000)