While there have been some good deals on the 6800's in the last few weeks, it certainly is a very good card even at $299. It sounds like you picked up the BFG 6800OC. I have the same card and really like it. As already mentioned, your card can potentially be "softmodded" using RivaTuner (available at Guru3d.com). Normally, the 6800nu has 12 vertex pipes enabled, and 5 pixel pipes enabled. Manufacturers disable the additional 4 vertex pipes and 1 pixel pipe using what's known as hardware masking. This is normally done due to these pipes not working 100% correctly. Since the GPU is the nv40 which is also the same chip in the higher 6800 models (although clocked slower), sometimes these chips have defects and therefore cannot use its full 16/6 potential for the high end card specs, so the defective pipes are disabled and put in the lower cards (in this case the 6800nu). However, in some cases the manufacturer must meet the demand for the lower cards, and use perfectly fine GPU's in the lower cards, and disable otherwise working pipelines. What RivaTuner allows you to do is bypass the manufacturer's disable of these pipes to see if they function correctly. This is called "softmodding" since it uses software to modify the card from its default specs. There is very little danger in doing this since you make no permanent changes in the card, and if the pipes are indeed defective, resetting the software to default settings relocks the defective pipes. However, if the pipes are functional and can be unlocked, you gain a performance boost, essentially turning a 6800nu into a mini-6800GT (hardware limitations such as memory speed rating on the 6800nu will keep you from hitting the 6800GT's actual specs, but performance will be close to the default 6800GT's performance).
If all this sounds too technical and you don't want to bother with it, there is another plus to this particular card. Many of the BFG6800OC cards actually were shipped with the 6th pixel pipe automatically enabled by default! My particular card was. So you may in fact already have a performance advantage over other 6800 cards right out of the box. And without softmodding, the 6800OC has a very good track record for overclocking, so if you are interested in that, you likely will get decent results. Personally my 12/6 6800OC hits over 10000 in 3DMark03 (good score for a 6800nu) after overclocking. Once the new version of RivaTuner is released, I'll try unlocking the 4 vertex pipes and see how the card performs.
So don't worry. You've made a good choice.