Thanks man, I thought since my PC is old. It would not be able to get better video cards but I can understand that better cards would just bottleneck. Final question, if I am looking for the dimensions of the card to make sure it fits, what would I look at to make sure that it will fit? The PC case dimensions? the motherboard dimensions?
open your case and measure your usable space from the inside. Don't just rely on the external dimensions. You'll have drive cages and wires to contend with on the inside. any card you get is going to be at least dual slot (stay away from triple slot. it's a waist of space in your case.) so make sure your case has enough PCI brackets on the back. From the pictures I found it looks like you should be fine.
In terms of the card, most manufacturers list the card length. try to leave yourself a little room though. If, for instance, you only have 12 inches of usable space in your case then don't get a 12 inch card if you can avoid it. you could run into clearance issues. Leave yourself an inch or two of wiggle room.
Remember you'll need space to run power as well. Depending on the card you get you may need anywhere from a single 6 pin to dual 8 pin. it's probably not going to be a problem, just something to keep in mind.
So chose the card you want, measure the inside of your case, get a version of that card that leaves you some wiggle room and get a power supply that can handle it.