That would not be a very good investment, IMO. I would wait for conroe, the G80 and R600 before upgrading. That's what I'm doing anyway. I know the wait will be hard, but in the mean time, save up money. If you get something now, it will be great for mabey a month or two, but when conroe, the g80 and r600 all come out, you will be kicking yourself for not waiting. Thats like what happened to me with the geforce 6800gt. I had a pci fx5200, and I desperately wanted a new card (and pc).
At that time, the 6800 and athlon 64s were top of the line, and they were looking mighty fine, and I knew the 7800 series and dual cores were just around the corner, but I wanted an upgrade so bad that I just said screw it and went with a 3700+ and a 6800gt. This was great for about a month; I was in the fast lane now, and I was enjoying it.
About a month after I got it, the GeForce 7800 series came out, as well as the dual core cpus. I was so mad, the 7800gt was cheaper than the 6800gt, and it kicked it's butt. The dual core cpus were also great. In the end I caved in and had to upgrade again to a 7800gt and opteron 165. The difference was a big one, and if I could do it over again, I would have waited, and saved a lot of money.
Trust me on this one, wait.
If you want to upgrade something right now, upgrade your power supply. It's a peice of ****.