They look like good selections in theory, however the parts you have select conspire to create a couple small problems.
I at one point had a very similar setup. The Sonata III does not have very good air flow for cooling, also the excellent EA 500 power supply that it come with has no fan on the bottom. With the P5K-VM you have a decent board, with an acceptable but not excellent CPU powersupply. I have a C2Q, so the problem was worse for me, but the CPU power supply components get very hot and the Sonata III provides very little cooling for them. With the stock intel cooler or other down draft CPU cooler you are OK as the power circuits get cooled by the CPU fan. With a tower cooler like the Thermalright Ultra or Xigmatek s1283 those circuits get almost no cooling.
If you are keeping stock cooler and not overclocking you should be ok, esp b/c the core2 duo uses less power than a quad.
Also along the lines of cooling the 4850 has a single slot cooler design that puts hot air into the case rather than out the back. Again the sonata III does not have the best air flow.
The other issues I see are
1) that the 4850 would probably benefit from a motherboard that has a PCIe 2.0 slots that run 2x as fast as the 1.1 slots on the p5k-vm.
2) that antec is now selling an EA-500D psu without telling anyone that it is not made by seasonic, but instead by delta. The Seasonic EA 500 is very highly rated, the EA-500D may be just as good or better but has not been properly reviewed or tested yet, and Antec has not made the change public.
I am still running my P5K-VM, and antec EA 500 psu but have changed my case to a coolermaster 690.
I want to get a P5Q-E. If you are still interested in the P5K-VM after you read the above... I would be willing to sell mine... It hits minimum 1600MHz FSB with a quad core up to 3.6GHz (3.2 Stable).
Nah, the OP doesn't need X38. Just pick up a P5Q Pro, and take a look at the NZXT Tempest. It has awesome airflow. With my friends build, (he had a E8400), the ambient case temperature was 29 degrees (Room temp 24) after 4 hrs of prime95. The processor was overclocked to 3.6Ghz.
Im low on cash, im sticking with the Antec Sonata 3... another question, since the 9800gtx and the 4850 are in similar price ranges, would the 9800 gtx fit in an antec sonata 3 case? and are its fans better?
After reading the posts above im choosing to go with a different mobo, as pci-e x16 1.1 ports wont fully utilize the 4850's power.