The Sunbeam is not too hard. I would say that it's easier to get right than the AF7, because the AF7 uses the standard push-pin system used by Intel, which is prone to failure/user error.
Once you have the kit assembled and the mount installed on the MB, the Sunbeam clips on similar to an AMD type of system.
That's a tough call on the video card. The truth is that everyone has their own comfort range for video cards.
The following is a bit wordy, but I wanted to provide some personal experience in hopes that it helps:
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I myself had an 8800GT, purchased earlier this year because I was not happy with performance in general from my 7900GT and because AoC was too much for the old card. 8800GT and 9800GT are the same speed, and I use a 1680X1050 monitor.
By November I was feeling the need to upgrade again. Nothing wrong with the 8800/9800GT, but for me, I feel like it hovers around the acceptable frame rate range. Good average frame rates, but a few too many low spots in shooters.
When I went shopping for a video card, I had two options: GTX 260 Core 216 (w/Far Cry 2) and 4870. The cards were very similar in speed, and I was interested in Far Cry 2, but I ended up with the 4870 because:
1) I own a crossfire board
2) The power demands of a 4870 are lower than GTX 260
3) I really have too many games to play already. WOTLK, COD WAW, Crysis Warhead, etc.
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The 4850 IS the best bang-for-the-buck card you can get. It's a clear step up from the 9800GT. You will probably have to turn down graphics in Far Cry 2 a bit more with the 9800GT than the 4850, but you could probably find some fairly satisfying settings.