The more cores the better, although you'd be wise to raise your RAM along with it.
TBH you shouldn't go above quad-core atm if you're planning to use your computer for heavy gaming, as you'd actually get better preformance from a dual-core (for the price) since most games are only sloppily optimised for quad at the moment, and the dual cores have higher clock speeds (per core). For Terragen though, quad-core optimisation will be better, particularly if you want to have photoshop or other CPU intensive programs open at the same time at a decent lick of speed. For RAM, I'd recommend 4 gig of the good stuff. Grahpics cards are useless for terragen, so don't bother going with anything expensive, the geforce 6 series will do you fine for that. Obviously if you're gaming you'll want 2x Geforce 8800 or something similar (seriously, I've got one Geforce 8800 and I get lag on Dx10 games, gone are the days when one of the latest graphics cards could handle max settings for years to come), just make sure you can cool it all down nicely.
I hope that helps. If it doesn't, then tough ;p.
