Why does every one wish to upgrade HDD in the Mac Mini, never mind the iMac's.
For what Apple charge to up HDD space its cheaper to purchased a FW800 external at 2TB and boot from that - they all run at 7,200 RPM, my god you can even put a Seagate running at 15,000 if speed is that important, not much of a boost though given they max out at 640G I think.
Best option, purchased mid tier MM and upgrade to i7 chipset, add external HDD and 8G of RAM, if you have cash, wait a few months and invest in SSD once price comes down - Mac Mini's are not too difficult to upgrade.