You won't be able to see DX10 graphics in XP. You won't have any problems running a DX10 capable card in XP's DX9 environment.
At the $80 price point, the Nvidia 9600GT beats out the ATI 4670 you have selected. As 1701E pointed out, avoid that Rosewill PSU. If you're on a strict budget, buy a cheaper case WITHOUT a PSU. Spend about $60-$80 (for your amp needs, might be after MIR) on a decent PSU, PC Power and Cooling etc. I use the same RAM you have selected, no issues. $15 more on that Hard Drive gets you 2x the space and a 32mb cache.
The E5200 overclocks well. You should be able to hit 3.0ghz on the stock cooler. If you want to go beyond that, look to an aftermarket cooler and some Arctic Silver 5.