If you want it to last, you might want to consider going with something like the Q9300 processor. It's only 2.5GHz/core, but has the 1333MHz FSB, and the newer smaller and more energy efficient design compared to the Q6600 you're looking at using. It is a bit more expensive though.
Other than that, I would just make sure to get the 64-bit Vista so you don't run into the 4GB RAM limit of the 32-bit version. That does mean you'll have to be a bit more selective about the hardware you buy. Things may be much improved over the 64-bit XP Pro days, but you won't be able to necessarily just go to your local big box retailer and buy any cheap bit of hardware that's on sale that week. You will need to first verify that there are 64-bit drivers for it. Kind of a double edged sword... On the one hand it's an added hassle, but on the other hand, a lot of the cheap junk that would just cause problems anyway won't have 64-bit drivers so those are largely weeded out just by circumstance.
Other than that... Make sure any motherboard you get can take DDR3 RAM, and if you go with DDR2 and a Core 2 Quad, make sure the RAM you get is some multiple of 2 of the total FSB speed. Either 1/4, 1/2, dead on, 2X or 4X the total bus speed. If you don't do this, you'll take a pretty big performance hit. And like I said, make sure it supports DDR3 for the future.
That's about all the more I've got.
After researching I've found that having a PC built versus buying off shelf is much better deal. Just wanted advice on the specs given to me by the person building it. Any suggestions/modifications? This computer is intended to last, be upgradeable, and will be used for complex GIS/GPS mapping.
Intel Core 2 Quad processor 2.4 GHZ
4 GB DDR2 Memory
500 GB SATA Hard Drive
ATI 3870 PCI-E Video Card
Windows Vista OS