Well, if all you plan on doing with it is gaming, I’d personally go with the FX-57. If you also do alot of media file conversion, video/audio editing, alot of programs at once, etc. you might prefer the X2.
Dual-core processors are really not necessary for gaming. the vast majority of games don’t multi-thread at this point.
If you want to see how all the various Intel and AMD currently stack up to each other, Tom’s Hardware just did a Benchmark Comparison of about every AMD/Intel CPU on the market at the moment here: http://www.tomshardware.com/cpu/20051121/index.html
You’ll notice the AMD FX-57’s outperform the X2s with games benchmarks like UT2004, Farcry, HL2, Doom3 etc., as well as many of the Application/Video/Audio benchmarks.
I will say that in the categories the X2 did pull ahead of the pack, they did so at a pretty significant bump. This is mainly due to the fact that the majority of current applications and even several benchmarks don’t support multi-threading yet. I wouldn’t worry about it not being useful in the future, however.
One alternate option is to go with a motherboard that supports 64/64-FX/X2 chips. Buy a single-core 64 or 64-FX now, and still have the option of upgrading to an X2 in a year or so, when the price will have come down, and perhaps more games/apps will support multi-threading at that point.
I’ll be putting one of these (haven’t decided which, though I’m leaning toward the Premium) in my next rig, (which I’ll be building in a few months): ASUS A8N-SLI Deluxe Socket 939 NVIDIA nForce4 SLI or the Asus A8N-SLI Premium nForce4 SLI
I see you only have one 7800GTX listed for video cards. You will need two absolutely identical video cards for an SLI setup. Do you already own a 7800GTX, or are you purchasing two at the same time. Remember the cards must be absolutely identical, including the BIOS of both cards.
I’m personally going to wait for more ATI Crossfire tech to hit the market before I make my decision to go SLI or not. I’m likely not building my new rig till March-Aprilish, so I’ve got time to wait a bit.
The thing I like about the Crossfire tech (in addition to the way it renders compared to an SLI setup), is the fact that as long as one card is a Crossfire Edition card, and the other one is a Crossfire compatible card, they’ll work together. Assuming of course, you have a Crossfire motherboard 
I’m only going to be doing one card in my new rig, and adding a 2nd card 9-12 months after I build, so I won’t have to worry about trying to find an absolutely identical (including BIOS) 7800-family card when it comes time to add the 2nd card. (I’m looking at the 16-pipe X1800XL or X1800XT. If you buy a Crossfire-compatible 12-pipe card like the X800, when you add a Crossfire Edition 16-pipe card, it gets throttled back to 12 pipes to match the older card.)
RAM- I don’t think you can use PC5400 RAM with any AMD chips. DDR2 RAM isn’t supported by AMD, only Intel, far as I know. You could go with PC400 if you go with the Deluxe Mobo, but personally I think it’s overkill. PC3200 is just fine, though I’d only get 2 gigs. (mark you, the Deluxe mobo will support up to 4 GB)
2 1GB sticks of Corsair ValueSelect PC3200 CAS3 will serve very well if you don’t plan on doing alot of overclocking. If you plan on overclocking, or you just really want to be ostentatious about it, you could get a 2GB kit of Corsair TWINX2048-3200C2 DDR400.
Anything more than 2GB, I don’t see you getting enough extra performance out of to justify the cost. You can always add more later if you’re unhappy. shrug
Hard Drive- You’re talking about the Western Digital Raptor 10,000RPM drives. shrug I don’t think they’re worth the money, unless you just want bragging rights. Get a small-capacity 7200RPM w/16MB cache Maxtor for your OS, and you’re not gonna notice the performance difference between the two.
I think the Hitachi 7K500s are a bit overdone too unless you really need all that storage space, but if you go with em, get the SATA 7200RPM w/16MB cache ones, not the Ultra ATA’s.
Optical Drives- shrug The Lite-Ons are just as good for considerably less, but if you really just want to dump money in this for the sake of dumping money into it, it’s your choice.
Speakers- shrug I don’t know diddly about audio, the Sound Blaster cards have gotten good reviews, though I think the X-FI Elite Pro is overkill.
Mouse- the G7 is a really nice gaming mouse, plan on picking one up myself.
Monitors- Try and get one with 8ms and you should be fine. CRT’s are still best for gaming, but LCDs have caught up quite a bit. Whichever model you pick, try and find user reviews of the model, you can see what gamers have to say about it. I’m going with a 19", the Hyundai ImageQuest L90D+.
One last note about SLI. I didn’t see you mention a case or power supply. If you’re going SLI, you want a big enough case for plenty of airflow, those two cards can put out alot of heat. I’d go with a full-tower case for better airflow, unless you’re really short on space for the box, were it my machine.
Also, make sure you don’t skimp on your power supply, I’d get a 600W at minimum.
You might want to look at components most reccommended for SLI by NVIDIA at their company’s SLI Zone