Although I totally recommend AMD over *ANY* Intel processor anymore, I don't think a 500 P3 is obsolete yet. I'd probably first try defraging the harddrive, setting typical role to network server, and making sure Ultra DMa mode is turned on IF you have an Ultra DMA/Ultra Ata Hard drive. (Look in my optimal performance settings for the procedures for these two.) Network server might pick you up a couple FPS, but if DMA mode isn't turned on you will gain maybe 5 - 10 fps possibly.
As far as the upgrade, all you'd need is a Motherboard, CPU, heatsink and fan for Cpu, and maybe some PC133 ram if you want top performance. You also *May* need a modem and soundcard if either are build into the MB and/or an ISA type. I'd probably also consider a video card if yours is not a TNT2 or better. (I'd prefer at least GeForce. ) You WOULD have to reinstall windows from scratch, which means you will need a windows 98SE CD. This does mean that I'd find a way to backup anything you want to carry over to the new system. You are quite literally getting a new computer, and just reusing your CD, floppy, harddrive, monitor, case, keyboard, mouse, power supply, maybe the ram, modem, sound, and video cards.
MBs I'd recommend highly are the asus a7v133 and abit kt7a. I personally prefer the asus, but the abit is excellent as well. As for CPUs, if you get the kt133a chipset boards, you might as well get the 133 Mhz fsb version of the 1 gig athlon tbird.
Once again though, I'd say it is too soon for an upgrade this severe. A p3 500 *should* be good enough for another 6 months or more. I just thought I'd tell you what's involved just so you'd know. I've done this kind of upgrade for my last three computers. What is also kind of neat about doing it this way is that for about another $200 you can buy another case and all the stuff you reused and end up with 2 systems by rebuilding a system using your old 500 P3. (This makes a great kids computer, or you could sell this computer and get over half your upgrade cost back potentially.

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bloom25
-MAW-
(Formerly of the)
THUNDERBIRDS