It's very easy to learn. Just look around in google searches. There are two very easy ways for the 2500+. First you can set the multiplier a little higher. Mine allows me to go up to 12.5x. If you leave the FSB (RAM) alone and set at 166mhz, XP will recognize as a 2800+
The second thing you can do is what I did. You leave the multiplier alone, (stock is 11x) and push the FSB forward to 200mhz. You need to have PC3200 or higher to do this effectively. When I loaded up at 200fsb x 11x multiplier, XP recognized this set up as a 3200+. It's running at 2.2Ghz.
To do this configurations, go into your BIOS and look under the Chipset options.
*note - if you do this and the computer is not running stable, try increasing the voltage to the CPU and/or RAM.
Hope this helps.
P.S. I'd go with the 9600XT 128. It's a beast. You really can't tell the difference between 128 and 256. Maybe unless you're playing Doom 3 with the rates maxed out. But if you're going for a 256 card, you might as well just get the 9800XT or the 5950 Ultra.