That old of a machine, I think you have no hope in increasing performance to race online in all honesty. Like I said, my PC that is broken at the moment was a P4 2.5 with only 8 mb shared onboard graphics. I ran it with the bare minimum graphics options ingame and was able to get around 35-45 FPS. I upgraded from the 8 mb onboard to a pci 256 mb card (quite a difference) and gained absolutely NO frame rate increase. The only thing it allowed me to do was "pretty" it up, ie, switched from 16 bit to 32 bit, maximize ingame settings, and run AA/AF on the card. I still get the same frame rate at 35-45 FPS fluctuation. If I turn all the settings down, I still get the same frame rate. The card helped me in looks, but not in performance. In my system, I am now bottlenecked with the P4 2.5 (which runs at 533 MHz bus). With a separate graphics card in your machine, you may be able to run at a higher resolution or with some graphics settings turned up higher, but I'd say you will be probably getting the same frame rate as you are now.
20 FPS offline on a P3 600, I'm stuck now with the old Celeron 667 with 512 MB ram and I installed the 9250 card in it to see. I'm getting a lovely rock solid slide show of 5 frames per second. Though it looks to me more like 5 frames every now and then, rather than per second, LOL. Can't wait for the tax return check when I can pick up my Athalon 64 3500 with a gig of memory that I'm looking at. I've never had an up-to-date computer. The last computer (and first ever) I bought was an original Pentium 75 with a whopping 8 MB of EDO ram back in 1995. Big oversized 2.1 Gb harddrive. All other's have been hand-me-downs. I've had 8 computers in the last 10 years between my purchased PC, 5 scrap PC's from work (to scavenge ram, drives, network cards, vid cards) and 2 hand-me-downs. I just threw away 6 PC's about 6 months ago. I gutted them, so I have a TNT 4mb, S3 4 mb, nVidia 2D, and an ATI RAGE 128 16 mb cards laying around, but believe me, they won't do you any good (I tried). Got a drawer full of processors as well, P75, K2 266, K2 233, couple of Pentium Overdrive 125's, and now a P4 2.5 that may be junk (not sure if it's the board or the processor). I don't know why I kept them all, since they're all old junk nowadays. I tend to keep all my junk for some reason.