The E4300 and x2 4600+ are very equal in performance...and the 350 watt powersupply will normally provide enough power for the system you mentioned...

the choice between these 2 processors depens on your use of them: if you want to save energy, go for the Extreme Efficient cpu of the X2 4600+...it uses the smalest amount of energy in idle mode...

If you wanna get maximum performance out of your cpu by overclocking your cpu...buy a better motherboard together with the e4300...then you can usualy get a 90% more overclock than normal speeds, only on air-cooling.

The best motherboards to get a high overclock together with the E4300 are based upon the new nforce 650i chipset (or the more expensive 680i chipset) or the intel P965. But, most of the times, these chipsets are a bit more expensive...

The second thing you have to realise is that not all s775 motherboards will be able to support the new core 2 duo processors...the p4m800pro-m does not seem to support the e4300..

I hope my comment helpted you a little and take care,

the poor student