Try and get a better PSU than you absolutely need, then you have headroom for upgrades.
As for your questions . . .
1. The 8600 is a great choice right now, it offers really good performance and DirectX 10 at a decent price.
2. The FX5200, even though it's an old card, will perform better than your onboard graphics*. It also won;t use your system RAM, so your overall performance may increase too. But don't expect it to work wonders, it's still an old card (so don't expect to go and play Bioshock or Crysis with it).
* - how do I know? I'm not 100% sure, but I had integrated SiS graphics (worse than the 6150 by a fair way) and installed an Nvidia Vanta. I haven't tried 3D games, but the overall performance went up slightly and the system seems more stable.