What you have in mind is going in the right direction, but the components are a bit outdated. You should go with a newer "Sandy Bridge" Intel CPU, like an i7-2600k and that needs an LGA1155 socket motherboard and I'd recommend the Z68 chipset or newer. That combo will be similar in price to the i7 950 but faster and use less power.
Then get as much RAM as you can afford. 16GB is a good target, and fairly cheap these days. If possible, and if you expect the machine to last for a few years, get it with 2x8GB DIMMs instead of 4x4, then you can upgrade to 32GB in the future (and make sure you get a motherboard that can support it).
The graphics card doesn't matter that much for Terragen 2, it's not used during rendering, only for the 3D preview object display and node network view, so you just need something fairly capable in OpenGL with good drivers. A Quadro is overkill. A decent "gaming" card will do fine, and give you more performance for the dollar. That being said if you use other 3D apps they may prefer a 'workstation' card with certified drivers like a Quadro, and that particular card seems to be fairly cheap (largely because it's pretty old). A machine I bought about 4 months ago has an nVidia Geforce GTX 570 and I'm quite happy with it. It runs about $300 now, so a bit more than your Quadro, but you could step down to the 560 or other options to save some money if need be. Tom's Hardware's gaming graphics card list is usually a good reference:
http://www.tomshardware.com/reviews/gaming-graphics-card-review,3107.htmlLooks like they're recommending the AMD Radeon HD 6870 in that price bracket.
- Oshyan