You need to know that there are 4 different Athlon/Duron versions.
I do not know your exact motherboard specs, if I did I could tell you for certain which CPUs you could use. What board do you have?
The first and original Athlon was a Slot, rather than a socket. No Durons were ever made using Slot A, so this is not what you have.
The second version used Socket A and had a FSB of 100 MHz DDR (200 MHz equivilant). Your Duron is of this type. If this is the only type of Athlon/Duron you can use you are limited to a 1.4 GHz 200 MHz FSB Athlon model, or likely with a bios upgrade, any Duron. This version of the Athlon used the Thunderbird core and the Durons at 950 MHz and below used the Spitfire core. Durons above 1 GHz use the Morgan core.
(There are 200 MHz and 266 MHz FSB Athlons at 1200 MHz to 1400 MHz. All Athlons above 1400 MHz are Athlon XPs)
The next Athlons ran at 1200 MHz, 1333 MHz, and 1400 MHz with a 266 MHz FSB. These were the 'C' version of the Athlon. It's possible you could use these, but I need to know what board you have to tell you that for certain. These Athlons were also based on the Thunderbird core.
The most recent Athlons are the Athlon XP series of CPUs, which start at the model 1500+ and currently run up to 2200+. (True clockspeeds of 1333 MHz up to 1800 MHz.) Your board *might* be able to take these, but again I'd need to know what your exact motherboard is. The model 1500 up to 2100 use the Palomino core, and the newest Athlons (2200+ is the only one currently) use the new Thoroughbred core.
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Judging by your current CPU, I know that with a bios upgrade you could probably use up to a 1400 MHz 200 MHz FSB Athlon. Without a bios upgrade you are probably limited to a 1200 MHz 200 MHz FSB Athlon.
Regardless of what you do, your gaming performance will be limited by your video card with a > 1GHz CPU, but your overall system will be significantly faster.
