Get only the :
1. Win 7 Home Premium x64 is enough for you
2. Samsung F3 7200 rpm series HDD
the rest can wait untill the Bulldozer, Sandybridge, and the AMD/ATI HD6000 are there.
If you can not wait :
1. Yup HD5850 is the correct choice
2. An upgrade to X4 965 or X6 1055T
3. An SSD is also not bad
Or if your PSU is still enough...a second HD4850 is also not bad.
This would mean having to stick with Windows XP until you can afford the Windows 7. The GTX460 1gb should be plenty with your resolution with the added bonus you can SLI a second in the future for excellent performance.
You could see if anyone would buy your current computer, with the money from that you may be able to get a second GTX 460 straight away, or the Windows 7 plus money saved towards a second GTX460, or if really lucky both.