It is BTX, but I wouldnt go jumping on an aTI 3000 anything, unless you want to get a nice power supply for your rig. This is the best one for a 5100, good for 100 bucks, very good.
470 watt
The nvidia 8600GT is pretty well beyond that power supply's capabilities also, the thing isnt going to peak much higher than 340 and the 8600 recommends at least a 350 watt.
Also Lagrange has to consider that those source games he bought are pretty cpu dependant so theres so sense splurging on a high end graphics card that his cpu won't keep up with. That dell motherboard is not going to get along with anything but a Prescott pentium4 cpu.