Are you dutch as well Dave? Asking this because of your username. If so I might have some good advice on places to be, and not to be.
The i7 is a logical and best choice for TG if you'd ask me. Getting about 8GB RAM would be nice as well, also for your Maya and C4D work.
Terragen won't benefit from a good videocard for rendering, but if you need a good one for Maya and C4D you might go for something of the 260-range from nvidia and if you have some extra money you could always look for one of the quaddro-series which are told to do very well with Maya, C4D and similar apps.
Also, if you're looking for a really cool and fast pc I'd also go for Solid State Disk. OCZ has some very good ones in their Vertex-series. 2x 30GB in RAID 0 of them will make your computer start up at least 4 times faster (for example). Not cheap at the moment, ok.
Martin