6800Gt Dual SLI or X850 PE

First post and a bit of a newbie with this building PC's lark but have decided to have a go myself. I think I have most of my specs sorted (below)

Asus A8N-SLi Deluxe nForce4 SLi (Socket 939) PCI-Express Motherboard (MB-087-AS)
OCZ 1GB (2 x 512MB) PC3200 Dual Channel Platinum Series EL-DDR REVISION 2 CAS2 (MY-048-OC)
OcUK GeForce 6800 GT 256MB DDR3 TV-Out/Dual DVI (PCI-Express) - Retail (GX-029-OK) x 2
AMD Athlon 64 3500+ Winchester 90nm (Socket 939) - Retail (CP-101-AM)
Creative Sound Blaster Audigy 2 Value 7.1 PCI Sound Card - OEM (SC-027-CL)
Western Digital Raptor 74GB 10,000RPM SATA 8MB Cache - OEM (HD-015-WD)
Western Digital Caviar Special Edition 250GB 2500JD SATA 8MB Cache - OEM (HD-010-WD)

However I am wondering on the GFX card front what to go for. the one nightmare I am having is do I go for a dual 6800 GT SLI combo or just straight for an X850 PE? I am really unsure as to whether SLI will take off or die a slow and horrible death and whether the dual 6800 GT's will outperform the X850 on current games. I play mostly HL mods and HL2, all at low setting screen size wise for matches.

There seems to be so many conflicting views out there and would really appreciate folks views.

SLi is nice in HL2, definitely a big gain. If you're only planning to run low res it's not worth it, however moving to an SLi setup would allow you to run higher res without compromising framerates for matches. Dual 6800GTs will outperform a X850 in HL2, HL it's not going to make a lot of odds - it'll be stupidly fast on any modern card. Other games will vary depending on how tuned for ATI vs nvidia they are and whether they can take advantage of SLi.