picking hardware

When looking at hardware for a mining rig, I assume we look at 3 things, speed of the GPUs, cooling and power supply/consumption. I figure that everything else doesn't matter as much (bus speed of the motherboard, CPU, RAM, hard drive capacity, etc). These probably have a factor, but the as long as the over all system can run the card to full efficiency, you're good to go?

I ask, because I have a desktop that's just sitting around. I can easily upgrade the video card to a mine-worthy card and at least kick it off for a minimal investment. Granted, I'm not expecting huge speeds, but it's better than running just the CPU.

The other option (for an additional $350) would be a mining contract. Thoughts? I don't know if I can convince my wife of this new 'electronic money' and just dump $500 into it.

If you have a shovel, may as well start digging, right?

Thanks,