You said that 2 gx280s would be the best, now you say 1 can perform better at some times.
If you read what i wrote earlier, I advised against SLI or Crossfire. Then you asked me what is the best possible setup. Well I said that it is 2 GTX 280, but I also added that in some cases a single GTX 280 will perform better, but I do not personally believe in SLIing or Crossfire. It helps with very big screen and resolution, but it doesn't always help. Please go have a look at Tom's Hardware charts and try to compare the frames per seconds you can get with this and this card. With a single GTX 280, you will have an amazing computer that plays just about any game great. Crysis is a special case, as I find even now with the best hardware, it seems like it won't run AMAZING.
I understand you want help because you lack knowledge, but here are a few basic advice.
1. Don't SLI, don't Crossfire
2. For gaming go for Core 2 Duo, for HEAVY multitask go Quad Core
3. Don't plan on overclocking yet, get some more knowledge first.
4. Get the best GPU you can now and upgrade to a better one when this one doesn't suit your needs anymore.
5. Stick with DDR2
6. 2gb RAM is a good enough, 4gb is great, 8gb isn't used in 99.9% situations.
7. Don't go cheap on the PSU (You already got that)
8. Getting 2 Hard drive can be interesting, either to have your o/s on 1 and the rest on the others or to run them in RAID.
9. To overclock or to leave room to overclock, use intel based motherboards, such as P45 or X48(they are a bit pricy tho)