Would a SSD help the situation?
I just ordered 4GB of RAM and a 80GB Intel SSD. These SSD are $219 now.
IMO a total waste of money. You can get a 1TB 2.5" WD for ~$200.00 and 640GB for less.
I just installed one for a client in his new MBPro yesterday (1TB).
A cheaper and slightly slower option would be an external 3.5" FW 400 enclosure with a drive size of your choice (2 TB even) as your startup drive.
The EXT FW 400 is not that much slower than the SATA in the real world for your intents and purposes, plus as said before by Crimson, if you keep it all local then all the better.
If you want a NAS then get a "bad" Time Capsule, they are easy to repair and being an Apple product simply works better together with the Mini. The only thing slowing you down in this case is the lack of wireless "N" in the Mini but thats no biggie either.