I'm still one of the few "gamers" ... well .... light gamers with an awfully old system. It consists of:
AMD T-Bird 1.2
Gigabyte 7ZX Motherboard (based on the Via KT133)
512 PC133 Ram
GeForce 2 mx400 64MB by Elsa!
Now, a couple of weeks ago I bought a Radeon 9600 Pro by Sapphire and .. WHAT A DIFFERENCE IT HAS MADE!!!!!!
With the same system I'm getting great performance in every game and benchmark I throw at my system:
With the old v/c 3dmark2001 --> 2300 3dmarks
With the new v/c 3dmark2001 --> 7900 3dmarks
With the old v/c 3dmark2003 --> n/a

With the new v/c 3dmark2003 --> 3000 3dmarks
With the old v/c Aquamark 3 --> n/a

With the new card Aquamark 3 --> 20,000!
Great performance also in UT2k3, RTCW, etc...
So Why did I post this?
If you are like me and have an old system but you definitely don't want to spend $250 for a new motherboard + cpu combo plus $150 - $350 for a new videocard, just get the videocard.!
Although your cpu/motherboard will be the bottleneck, you will be assured great performance and if in the future you decide to upgrade your cpu/mobo your system will be even faster.!