A guy at work gave me a PII last week. It only has a 2 gig HD. I've got MAME running on it now, and was amazed at how many games would fit.
I plan to make a 'countertop' arcade w/it that I'll put in one of my kids rooms. It's not too bad, but as expected the more graphics, sound or CPU intense games don't work or don't work well.
Go ahead & start your cab. I waited pretty much til the end to paint mine, that seemed the smart thing to do. If later you can scrape up a better PC, just swap out the 'ole PII & put the new one in there.
Good luck!!