I'd go with the 2 6800GTs. While neither is quite as fast as the 7800GT, the two of them combined should run circles around a single 7800GT. I would expect them to be able to give acceptable performance for a longer period of time than a single 7800GT as well, so ultimately giving you a little more life out of the system for the money.
OTOH, you could always go with just the single 7800GT today, and then in a few months when the prices are significantly lower, add in a second one. I can't claim to know anything about setting up an SLI configuration from a single card and how easy or hard it will be, so if this seems like an attractive idea, I'd look into that aspect of it.
Alright well, I am building a new system, as mine is out of date, so I was just having some opineons about it (system will be used for gaming, such as HL2 and Battlefield 2)
CPU:AMD Athlon 64 X2 Dual Core 4400+
Motherboard: ASUS A8N-SLI Premium AMD socket 939MB
Graphics Card:
2 X ASUS 6800GT 246mb = $599.80 or
2 X BFG 6600GT OC = $438.8 or
BFG GF 7800GT OC 256MB = $429.95
Harddrive: Maxtor 160GB Sata
RAM: OCZ 512MB 400MHz Gold 1gb
Drives: DCVD/CD RW LG Combo
Now I am pretty confident with the mobo, CPU and other aspects about the computer that I have chosen as I have already read many reviews, the only thing that i am having troubles is with the decision of the graphics card set up. The mobo that i have chosen has SLI technology so I was just wondering if should take advantage of it with two vid cards with 6600GTs or 6800GTs, or just put one powerful one in it, in this case it will be BFG 7800GT. I have also shown the prices that I will be paying (local shop that I trust very much), which is another factor. Anyway, any opineons on what I should do would be greatly appreciated, thanks!!

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