I'd go for the 1 Ghz unless you really want to conserve funds... IMHO, the new $1000 emac is one of (if not the) best value I've ever seen in a mac.
That's the model I'd go with if I were you. $1000 for a 1 Ghz G4, combo, 60 gig HD, with Radeon 7500 and AE ready... that's so sweet. You need to upgrade the RAM of course, but that's cheap as long as you don't get it through apple. IMO you'd see a major performance difference between this one and the old generation 800 mhz.
This machine could last you quite a while if you needed it to, so you might not feel the need to run out and buy a PPC 970 Powermac right away... you'd be able to wait for the second generation and/ or price drops because the emac would be up to any task for quite some time. Just my opinion.