Banana_Joe :
The 6850 consumes 127watts at max. 300 *real* watts should be enough if the computer still have factory setup.
Chances are any power supply with 300W is not going to be able to power a system like that. It might, if it was 80 plus and had some of the best efficiency in the world, but that is probably not the case since the OP has not messed with the case other than the upgraded graphics card.
Compaq is made by HP, which generally has power supplies that have 16-18a on the 12V rail.
If it had 16a on the 12V rail, using P=IV, the rail should support 192W. The 3800+ is an 89W processor.
127
+89
216W at full load. And we hadnt even factored in efficiency yet. If the power supply had about a 70% efficiency, which is about what it should have, it would put out about 138W.
I would highly recommend a new power supply.