Aperture, in it's current state, requires a powerful graphics card more than anything. 🙂
That said, as a heavy user, I'd say ram AND the proc will help. I just moved from a 2.3Ghz G5 Powermac with 4GB ram, to a 2.8Ghz iMac with 4GB. I don't really know how the lower end machine, 2.4 with less ram, would run, but I can say the 2.8 is more than satisfactory, and considering Aperture is due for an update, or at least I hope, I'd say go for all the machine you can afford.