I run a small business also where I must make capable computers under a tight budget. (For instance my new builds cost under $400.)
A new mobo and processor will greatly help your overall system speed at the least cost, and you can get one that will utilize all your current hardware. Good motherboards make everything run faster, and I always buy motherboards that have the potential to upgrade to another cpu later without having to change anything else.
For under $120 dollars you could greatly speed up your system. I did some research for you as to what I would buy if in your situation. Here's what I'd buy...
The 3.0Ghz dual cpu will come with a heatsink and fan, so all you need to do is remove the old mobo, install the new one, use your old ram, hook up the cables, and boot! (of course, you will need to install the driver disk that comes with the mobo.)
You can also later upgrade to a PhenomII Quad-Core processor without buying anything else as this motherboard will accommodate faster processors.
Let me know your thoughts. I try to logon everyday, so feel free to comment. Thanks.