On a performance per dollar basis, the core i7's are still a little bit too pricey relative to the Core 2 Duo/Quad lineup. Not only in terms of the CPU, but also the socket 1366 Motherboards and the DDR3 RAM. Unless you are the type that must have the absolute latest and greatest, I would save some money and get a high performing C2Q/775 MB, and you can spend that savings elsewhere such as upgrading to a quieter case and PSU.
I would personally go with one of the Core 2 Quad's, a regular P45 chipset motherboard such as the Gigabyte GA-EP45-UD3R. Overclocking is extremely easy with a MB like that.
Most benchmarks have shown that the performance gains are huge from 1GB to 2GB to 4GB RAM, but going from 4 to 8GB has less of an effect for most use. If you are doing substantial amounts of video encoding though, it may be worth it to you.
The 9800 Videocard should handle anything you want to throw at it. I admit I don't know a lot in this area as my gaming is via console.
If you want a quiet case, you might consider the Antect SOLO instead of the Sonata III. The SOLO is much more quiet, but is a little more expensive. It doesn't come with a PSU either, so you would have to pick up one of those as well - the Corsair HX and VX lineups with 120mm fans are extremely quiet and high quality.
I just built a rig after Christmas with the above mentioned MB and a C2D E8400. Overclocked the CPU from regular 3.00 GHz to 3.78 with zero effort. Cool and quiet.