The power supply could be a problem but depends on what other components you are running. A good GPU for your system would be a 6600GT or like you say an X800. I believe that doom works best with ATI so go X800.
I wouldn't upgrade without making a big enough jump. A radeon 9800 pro does great in HL2. far better than in Doom3. With a 9800 pro, I played through it at 1280x1024 highest details, but without FSAA enabled. To me, you need a X800 pro, X800XL, 6800GT caliber card to really make a big difference, and you will want a new power supply with those. Until you can buy one of those, try OC'in the 9800 pro if you need a few more fps.
If you are in the USA or Canada, ATI has a refurbished X800pro AGP with 3 year warranty for $180(USD) + shipping in <A HREF="http://shop.ati.com/product.asp?sku=2700726" target="_new">their clearance section</A>. Otherwise I'd suspect that 6800GT and X800XL will drop to under $200 by years end. Just remember that in some games you will be cpu limited.