do i need more power

The only reason you may need another power supply is the motherboard you purchased is an SLI M/B, if you plan on going SLI [dual video cards] and the P/S you have doesn't support SLI setups, or have all the M/B connections you may need, then you would need another P/S, but for your listed setup 430w is sufficient.

You may need to buy a more powerful PSU.

Here's a breakdown of your PC power consumption:

Athlon 64 X2 4800+ (not overclocked)...................96w
GeForce 7900GTX (non overclocked)..........................95w
Motherbaord............................................20w
2 Sticks of RAM.......................................10w
Hard Drive..............................................20w
DVD Drive..............................................15w
Case Fans (assuming 2)...........................10w

Total Power Consumption @ 100% Load...266w

Estimated max load while gaming............250w

Chech the 12v rails, the typical modern PSU has at least 2 of them. Add up the amps on the 12v rails. If the total is less than 24 amps then I suggest you buy an new one. If you are going to overclock then you definitely need a more powerful PSU.

Sorry, I don't know anything about watercooling.

Yes, the NeoHE 430 can provide up to 384w on the three 12v rails.

However, I am still reluctant to recommend it because of the problems the previous versions had with Asus motherboards. Yes, a new revision has been released in Febraury that resolves the issue. But it is still possible that older revisions are still sitting in warehouses waiting to be shipped.

The OP wants the Asus A8N-SLI Premium.

Yeah, a water cooling system definitely wouldn't consume enough wattage to put stress on a 430W PSU if you're only talking about the other components consuming 250W.