That's hardly a computer that would entitle someone to call themselves "King Gamer" lol.
C-Man is correct about the the HD 3850 and HD 4670 as the best AGP cards out there but jeez, those cards cost over $100 each and won't perform that well because AGP bottlenecks them. Also, your 400W power supply is not strong enough for the 3850, it's only strong enough for the 4670. This system upgrade would be a better option:
CPU: AMD Sempron 140 2.7GHz - $33
http://www.newegg.com/Product/Product.aspx?Item=N82E16819103698
Motherboard: Foxconn M61PMP-K AM3 - $47 (-$7 mail-in-rebate=$40)
http://www.newegg.com/Product/Product.aspx?Item=N82E16813186189
RAM: 2GB (1X2GB) Crucial DDR3-1333MHz - $50
http://www.newegg.com/Product/Product.aspx?Item=N82E16820148221
Total Cost: $130 (-$7 mail-in-rebate=$123)
This would completely change your computer to a platform that is fully up to date with a much faster CPU and RAM. It would also give you a PCI-Express v2.0 slot which is key because:
ATi Radeon HD 4670 1GB AGP - $123
http://www.newegg.com/Product/Product.aspx?Item=N82E16814161284
So the slow AGP version of the HD 4670 costs as much as the full system upgrade by itself and would be completely bottlenecked by not only the AGP slot but your ancient 32-bit Intel Pentium 4 2.4GHz CPU. In addition to that, no card weaker than an ATi Radeon HD 4870 or nVidia GeForce GTX 260, even in PCI-Express v2.0 form, is powerful enough to play games in situations where it would use more than 512MB of RAM. Now, check THIS out:
ATi Radeon HD 4670 512MB PCI-Express v2.0 - $63
http://www.newegg.com/Product/Product.aspx?Item=N82E16814141114
The PCI-Express v2.0 card, which has EIGHT TIMES the bandwidth of AGP 8x, is about HALF the price of the AGP card with the same GPU. This card would slaughter the AGP card and if you had the CPU I recommended which is a 64-bit chip at 2.7GHz, the card wouldn't get bottlenecked! The total to actually turn your computer into a system that can actually play real games is $193. Any money you spend on one of those AGP cards will be a complete waste! Look at it this way, you can spend $123 and still not be able to play anything properly or you can spend less than 50% more and not only play games properly but have a system that will feel like a total rocket to you. You will have more than DOUBLED your CPU processing power and more than TRIPLED your memory clock speed. Not a bad way to spend $60, definite bargain eh?
Intel Pentium 4 2.4GHz:
Passmark Score: 322
Passmark Rank: 740th in the world, listed near the bottom in low-end CPUs
http://www.cpubenchmark.net/cpu_lookup.php?cpu=Intel+Pentium+4+2.40GHz
AMD Sempron 140 2.4GHz:
Passmark Score: 887
Passmark Rank: 443rd in the world, listed at the top of low-mid-range CPUs
http://www.cpubenchmark.net/cpu_lookup.php?cpu=AMD+Sempron+140