Re: Could I do this...
If you could OC your 3800+ X2 a bit then I don't think it would create much of a bottleneck. Personnally I think you should spend the money on bringing your system more up to date and making it more upgradable as you already have 2x7800GT. My recommended upgrades would be an AM2+ mobo, DDR2 memory, and a new CPU (perhaps a B3 Phenom?). You could definitely get that and more for the $600 that the 9800GX2 would cost.
And BTW, if Nehalem is going to change the industry, then it's already been done. AMD have been using Hypertransport in place of the FSB and an integrated memory controller for years.
AMD Phenom II X4 955 BE @ 4.0GHz + TU120E lapped - MSI 770-CD45 AM3 - 2x2GB OCZ DDR3 1333MHz - Sapphire HD 4870 - Samsung Spinpoint 500GB 7200RPM 16MB cache HDD - Tagan TG600-BZ Piperock - (Currently open test bed) - Windows Vista Home Premium 64bit.