I'll repeat my post from a similar thread...
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The first thing I'd do is look at the specs for each board - there are differences in the number of PCI and PCI-E slots (and available slots with one or two double-slot-wide VGA cards installed), firewire support, eSATA, etc...
650i Stats Comparison
...caution - some of those listed stats are off and some are 'marketing hype', but for example, if you plan to run SLI with 2x-wide 8800 cards installed...
both MSI boards leave two PCI and one x1 PCI-E open
the ASUS board leaves one PCI and one x1 PCI-E open
the ABIT board leaves one PCI and two x1 PCI-E open
the ECS board leaves two PCI and one x1 PCI-E open
Rear panel connections:
serial/COM: ECS only (other boards may have on-board header to add a cable to)
printer/LP1: MSI boards and ASUS only
eSATA: ASUS and MSI Platinum only
firewire: both MSI boards and ASUS only
S/PDIF: all boards have some form or another... ECS and MSI Platinum have both.
Chipset cooling (arguably from worst to best):
ASUS - heatsink on NB, but NO heatsink on SB or MOSFETs
MSI SLI-FI - heatsink on NB and SB, but not on MOSFETs
ECS - heatsinks on NB and SB, with fan on NB, no heatsink on MOSFETs
ABIT - heatsinks on NB, SB and MOSFETs
MSI Platinum - heat-pipe cooling across NB, SB and MOSFETs, plus optional fan included for NB.
(NB fans are small/high-rpm = loud... so that may swap the ECS and MSI SLI-FI positions. It's optional on the MSI Platinum heat-pipe setup, but may be needed on the smallish ECS solution)
Other:
ASUS - inferior audio solution, only 2 PCI slots, narrow board only uses 6 stand-offs for support
ECS - it's primary physical x16 PCI-E slot only ever works in 8x electrical mode (never in 16x).
ABIT - has neither COM or LP1 rear IO connections, making it the worst for legacy support.
...obviously there are other layout differences between the boards as well (IDE/SATA/Floppy connector locations and orientations, etc) - some better than others, depending on your case and other hardware in use. There are also differences in bundling, but those should be secondary considerations for the most part.
Related to the above, note that:
I only listed the physical specs/features of the boards and nothing about support, BIOS versions, overclockability, user-experience, failure rates, etc. I'll leave that to you to ferret out . The intention is to first narrow down your choices based on what configuration best suits your needs.
Again, I only listed the differences... so the fact that the ABIT board is missing both COM and LP1 rear panel connections may not mean anything to you in your own personal ranking system if you don't need those connections.
...the 'best' board for me is the MSI SLI Platinum - but that's due to my own needs and ranking system.
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...as an addendum, Gigabyte's 650i board just hit the market... it looks like a very nice setup and has some nice BIOS features (dual BIOS, save up to 8 profiles, built-in flash utility). My only reservations about that board is that it hasn't been reviewed yet - no idea how well it'll perform or be supported and it has no eSATA. Having said that, it would be my second choice of 650i boards (after the P6N Platinum).
If you're interested in more info on the P6N boards, we've got a 37 page thread going on
over here.
BTW, the P6N Platinum has been run with a 533 FSB in
Coolaler's Forums(Taiwan), it also has 2 PCI-E slots for SLI and also supports 1333MHZ FSB for future cpus.