Tualatin celerons with an adapter are probably 'as fast as you can go', but they still aren't 'fast' (similar to coppermine P3s). Your celeron is probably the older model and won't do 750mhz, but that's worth a try, and after that you have to ask yourself what a ~1ghz PC is worth to you in excess of what you have - it will still run XP, play some (but not all) videos, and surf the net, but neither system is going to be fast.
Either way, your main obstacle to upgrading will be finding parts, rather than cost; I've pulled (and used) faster chips than that from PCs sitting out on garbage day!