You didn't state what you do primarily on your PC, so what I have to say may be off a bit.
Would help to know your OS also, as 4GB of ram will not be completely recognized by a 32bit system.
Hope this helps a little.
1st; will 200 pound cover all three of those parts?
2d, you did not post your power supply specs. ie. brand, model, wattage, +12V amperage. The 7790 requires less power then your 8800GTX (http://forum-en.msi.com/faq/article/power-requirements-for-graphics-cards) BUT the age of your power supply and it's manufacture will have some effect (perhaps significant) on it's ability to power the new system.
3rd. The new motherboards, processors and RAM have significantly improved features over your current equipment. You obviously know this or you wouldn't be looking at upgrading, but your planned upgrade, IMHO, is more like an update and (other then the 7790) you are using outdated parts.
Yes, you will see some improvement in performance, and you will theoretically be able to play using DX11; but the bottle neck in gaming MAY then be your processor- even with 4 cores.
Your system is similar to one I had; when I went with a 4 core Phenom I saw some improvement, when I lost the 9800 cards (basically renamed 8800) and went with GTX460's I saw even more improvment. When I changed two years ago to the 8120 8core and Sabertooth board, then to two 660's to replace the 460's - that's when I saw the biggest change.
I do know how the desire to upgrade and update can overrule common sense and patience; had it happen to me a lot! But I think if you can hold off and save for a new build you would be a lot further ahead and a lot happier in the long run.