NO NO NO! Again I say NO NO NO!. Your GeForce 3 is WAY WAY better than an FX5200. In fact the original Geforce 3 is better than a GeForce 3 Ti 200, but not the GF3 Ti 500. Are you getting poor performance in games? If not, i would keep it and hold out a few more months until the radeon 9700 drops to like $150 which should happens soon after Half-Life2 and Doom 3 are released. Trust me, your GeForce 3 will smother that FX 5200 128 MB in Doom3, even if the FX5200 is DX 9 compliant. Nvidia had to sacrifice alot of performance in the FX 5200 chip just to provide the extensions for DirectX 9. If you must upgrade now, what price range are you willing to go for, or do you want strictly bang for buck?
My OS features preemptive multitasking, a fully interactive command line, & support for 640K of RAM!