Just for a rough idea for that kind of budget:
- Athlon X4 - go with a true quad core for $100. Far as speed, go with the fastest Ghz. Most of them are within $100 anyway.
- At least 4GB of RAM
- Photoshop uses CPU more than GPU. I run Photoshop 7 on a 9500GT with no problems
Sorry I can't think of much more, but I'm dead tired. Hope this gets you on the right track.