I think the best bang for buck card would be a 9800Pro.
If you dont mind a refurbished videocard?
REFURBISHED: SAPPHIRE 100556-RD Radeon 9800PRO 128MB 256-bit DDR AGP 4X/8X Video Card - OEM $99.00
If you do?
SAPPHIRE 100556-RD Radeon 9800PRO 128MB 256-bit DDR AGP 4X/8X Video Card - OEM $126.00
( I could have said, "Get a Radeon 9800SE, for 80 bucks". But that card is slower and have fewer pixel pipelines. And as you seems to like shooters. I guess you some day buy F.E.A.R. or Condemned. And then you will be glad to have the power of the Radeon 9800Pro.)
Prices,newegg, webshop.
This card is able to play any game.
If you want something better? Then you will have to shell out about 150 bucks for the card. And probably a new powersupply for 70 bucks.
But first, download Everest home,and check if your motherboard is equipped with an AGP port? Check under "Motherboard" and "Expansion slots" for "AGP".
Everest home here:
http://www.majorgeeks.com/download4181.html
If you find,that your mobo does'nt have an AGP port?
Then this PCI videocard will be the best choise:
PNY VCGFX55PPB Geforce FX5500 128MB 128-bit DDR PCI Video Card - Retail $89.00
But dont confuse "PCI videocards" , with PCI Express. PCI Express, wont fit your motherboard.
You can find this cards at newegg, webshop.