300Ws enough for my PC?

If it is running fine now then there is no need at all to replace the PSU. There should not be any difference in power consumption between the NForce 2 mobo and the current one.
AREA_51

Nope, 300W should do fine with that rig.

My PC eats so much money that I'm in 'desperate' need of it to buy PC3500 RAM, help Svol with his OC project!
--- PM me for information.

If I plan on adding a RADEON 9700 would I need to go for 400W?

I think the rating of PSU (in term of watts) might not be a good way to determine if you system can handle some components upgrade. In the case of changing MB, it is ok to use your current 300W PSU since it is working fine with your current MB. However, the R9700Pro may load your PSU a bit more than the original Radeon. If your PSU is a good PSU, then it should be fine. Check to see first if your psu is a good one. If it is, say, Antec, Enermax, or Vantec, then you should be fine. If you can spend some money on a new good and quiet PSU, then go for it. The Antec True430 is nice and quiet and cost around $85 shiped (True380 cost a bit less and you might get it for $80 shiped).

Mocca

Nvidia or ATI, it's all depend on what you like and system optimization  

Can you tell us what the maximum amperage is for the 12V and 5V lines?

My PC eats so much money that I'm in 'desperate' need of it to buy PC3500 RAM, help Svol with his OC project!
--- PM me for information.