1. Almost all graphics cards in a given line are the same and you're really only paying for bundles or special programs that the individual manufacturer may have. Cards are mass produced and quality can vary. I usually go with the least expensive one that has the specs I want. The GTX 260 is a mid-ranged GPU, if you game a great deal, you may want a better card.
2. It is possible to get a motherboard that is overkill. Such as buying one that has four PCI-Express X16 slots when you only are planning on using a single or dual GPU setup. There can be overkill in terms of the CPU. Most gamers often spend way too much for the latest and greatest CPU only to discover that many of their programs cannot take advantage of the CPU technology. For instance, a user plays games that are dual core optimized, not quad core, and they are planning on upgrading again in two years yet they still buy a high end quad core CPU with a lower clock speed. The games must be optimized to use all the additional cores otherwise the value of the quad is wasted. Users who keep their PC's for several years can see the advantages of using a quad. I think most of the LGA 1366 motherboards are very overpriced, look for ones that have enough PCI-Express slots for your purposes from a good company. The GIGABYTE GA-EX58-UD3R
and the ASRock X58 Deluxe LGA 1366
might be a better value.
3. I agree with dpunisher on this one. From what I've seen, RAM pricing varies mostly based on speed and added features, like heat spreaders. Since you're wanting a low-end i7 920 system, not the higher end i7 950 or 975, you probably have the cash to burn on these features.
4. I own a Xonar D2 and a Xonar D2X sound card and enjoy them. Sound cards are only really necessary for people who listen or make a great deal of music or those interested in home theater setups. I refuse to buy Creative again because of the way they treat their customers.
5. Absolutely go for 64-bit.
For PSU's, I suggest Corsair, PC Power, Silverstone, and OCZ. You will most likely get a very good PSU with those companies.