I was in the same situation as you. From what I've read the 9800 pro will give you about ~6000 3Dmark03 marks, while the 6800 will give ~8000 3D marks. The lack of the extra 128mb of ram was kinda hard to get past, but I still think the extra performance of the 6800nu will outweigh it. I went with the 6800nu. (Just ordered it today)
As for your 2nd question, some of the 9800pro's are able to be modded to a 9800XT. There really isn't a way to check to see before you order it, but people seem to be getting good results. Either way, the 6800 will be better than the 9800XT or the 9800 pro.
You mention that time is an issue. the 6800's are still pretty scarce while the 9800pros are pretty common. You might not be able to get a 6800 before you could get a 9800. If time is really that important, the 9800 would be your best bet. However, if you can wait a week or 2 than I would go with the 6800.