@Philly: The price of the parts as listed comes to about $1350 with about another $50 for shipping.
@Hidinger: Thanks for the advice. I suspected that the graphics card that I picked might be more powerful than I need. I picked it based on the most recent Best Video Cards For The Money: Dec '08 article at Tom's Hardware. Tom's review listed the 8800 GT as providing "good 1600x1200 performance in most games;1920x1200 in most titles with some lowered detail". I had selected the Radeon HD 4870 because Tom's suggested that it could provide good 1920x1200 performance (without lowering detail). I'll be using a 24" widescreen monitor at 1920x1200 resolution. Aside from the Sims 3 I'll likely be doing some HD video playback (downloads at first but I'll probably add a Blu-ray drive once the prices drop a bit more). If you think that the 8800 GT can handle those then I'll go with that and save some cash.
As for Vista/XP, I can get XP Pro 64 bit at Newegg for $140 ($40 more than the version of Vista I had picked out). I know that Vista vs. XP is a debate that is still raging in the IT world. I've been using Vista on my personal machines for about a year and a half and have grown to prefer it. XP is still a great OS but I've been perfectly happy with the experience I've had with Vista. Plus, my wife (who would also be using the computer quite a bit) has a strong preference for Vista and I don't relish the thought of debating the issue with her. Anyway, I appreciate the recommendation and I definitely understand that you have more experience with and knowledge about PCs than I do but I'll probably just stick with Vista.
Thanks for the advice on the thermal paste. I have never done anything with processors before and that was the part of the building procedure that I was most nervous about. I've read some guides online but your tips offer some details I hadn't heard before. Thanks again.
Finally, in your third point when you said that a dual core processor would run the Sims 3 just fine, were you suggesting that my selection for the quad core Core i7 920 was overkill? If that's the case then I'm well aware of that. I've been holding off on replacing my old NetBurst based system until the true successor to Conroe came along. Now that Nehalem is here, I'm ready to jump into the next generation. Plus, since I'll be using the new LGA 1366 socket on my MOBO, I should be able to upgrade the processor for another several years down the line. I know that I could save about $300 and get amazing performance by going with a Core 2 Duo E8500 or Core 2 Quad 6600 setup but I have been looking forward to Nehalem for quite some time and that's probably the one component I'm most excited about.