Depends how urgent your needs, but just about every component will have some major revisions by end of June and some price drops. There will *ALWAYS* be newer stuff coming, but this is a pretty interesting set of months. This month both the nVIDIA 6800 and the ATI 800XT/PRO will be released. Rumors are the Opteron 250 will show up soon, etc...[EDIT: As of 5/4/04, guess they are both released]
Best question is what is your budget, based on that, you can start trading off the differences.
I suppose the fully qualified OpenGL cards are above your price range (ala nVIDIA QuadroFX and ATI FireGL's). They list around $2500 -- If your doing really serious 3D, I would check which applications are certified with the newer rev cards (the GL versions of new cards). They have better memory, shader performance for editing than the consumer cards are for gaming. [EDIT: Hmm...It had been a while since I looked, the FIRE-GL2-256T seems to be basically same hardware as 9800 XT, just with the certified drivers and a list price around $900. nVIDIA claims QuadroFX 4000 was some different hardware and has the pricepoint of around $2500 list]
The Thunder utilizes both memory controllers in both processors, so yes, it has more memory bandwidth, but the motherboard footprint is larger and more expensive. Keep in mind, if you run a single application that is not multiprocessor aware, you might not notice that difference.
WRT to 64-bit SATA, there is at least one commercial card out there, the RAIDCore RC4000
' ">http://www.raidcore.net/raid_controllers.html. They were just bought by Broadcom, so helps with the caveat about small vendors and their ability to release driver support. Silicon Image of course is developing 64 bit chipset as well.
I have a SCSI bias, growing up around Sun/SGI workstations, but I think I would skip SCSI and stick with SATA just on price/performance.
Other questions, can't give you much feedback...