Pretty much just did. Go for what you can afford.
How much RAM does it have currently? I mean, it's an AM3 board with a quadcore already in it. I don't think the program utilizes GPU hardware acceleration so getting a cheap used Phenom 2 X6 1055t (95w CPU) and maybe an SSD would be golden if the machine already has 8GB of RAM.
If not, then I'm going to guess it has 4GB of RAM, so getting 4GB more and a 120GB SSD could help too. That's looking at close to 200 bucks. After that I would grab a cheap GPU like a GT630 for 50 bucks.