Yea, I agree with the first post. It might be time to simply update your whole rig. You should be able to find decent setups on eBay, or on newegg, for complete towers for as little as $200, I know I sold my sister's old P4 Dell for about that much last year....
As for the drive, I didn't know they made them that small anymore. I mean, digital cameras have more memory than that nowadays. If you know there's an issue for 500MB, then don't push the barrier and 80 should be fine. But I can't even begin to imagine where you found something like that...
Just curious, what do you use ure PC for?
-AMD Opteron 185 ~ 2.6 Ghz (Thermaltake Typhoon heatsink)
-2 GB Patriot DDR2 RAM
-2x GeForce 7600GT
-Gigabyte GA-K8N SLI motherboard (F12 Bios Flash)
-520W Corsair PS
-320 GB Western Digital HDD
-250 GB Western Digital HDD
-80 GB Western Digital HDD