My suggestion is that you wait another 2 or 3 months before buying anything. I am not going to make any specific recommendations, just raise some pertanent issues.
It is clear that you have an existing system, which means that you can keep doing your thing for now.
Unless you are willing to deal with the unexpected surprises and glitches inherent in any brand new technology, I wouldn't recommend buying anything right now. New, high performance technology is called "cutting" or "bleeding" edge for a very good reason. Your posts do NOT indicate that you have either the experience or enthusiasm level required to operate on this edge. It can be EXTREMELY frustrating to be in that zone if all you want to do is play your favourite game(s) and use the system for any other routine things you do.
I am struck by the number of posters here that are focused on the hardware side, to the point they forget the object of the exercise is to run applications (games, spreadsheets, word-processors, etc.) in a stable, productive environment. In the context of your proposed application, gaming, this means a system that will provide a high enough frame rate to play the game, WITHOUT crashing on a regular and frequent basis, leading you to spend more time trouble-shooting the hardware than playing your game(s). Are you sure that is where you want to be? Buying Model 1 Mark 0 hardware guarantees that is where you will end up.
The reason that there is practically no support for SLI or Crossfire systems on the Intel side is because, until Conroe, AMD owned the performance equation in terms of gaming and almost all other applications. The only place where Intel performance consistently beat AMD was in audio/video creation and processing. This was because Intel deliberately designed the P4 to excel in this area. AMD chose to design their chips to be generalists. Funny how AMD tended to blow Intel away, at lower clock speeds too.
Now that Conroe CPUs significantly outperform current AMD product in all spheres, one can expect that support for features like SLi and Crossfire will show up fairly soon on the Intel side. But, you would be wise to step back a bit and examine what else you may wish to use your system for, other than gaming.
For example, if you are in a position to do a lot of video capture onto your system, you should be taking a serious look at ATI's All-In-Wonder video cards. They do a great job of video capture from a number of input sources, without the need for additional hardware. But if this is an issue, and you need to get the ATI card, you will need a system that supports Crossfire if you want to do the multi card/multi monitor thing. On that subject, you may want to wade into that pond slowly, both from a hardware availability and cost point of view.
I suggest that you consider getting the best MB with the most features with one video card and monitor first. Familiarize yourself with the features and quirks of the system, then move on to more sophisticated and expensive configurations. It is my understanding that the Apple 30" widescreen monitor is something extraordinary, both in terms of quality and performance (and price). This is the monitor that is included in the Tom's Hardware Ultimate Gaming PC ($10 K value). You may want check out the specs / components in this system to get some idea of what is available. That's assuming you have a minimum of $10 K to spend.
Many of the hardware-oriented posters here are into overclocking and other "tweaks" to boost performance by relatively small margins. You don't strike me as someone who is either interested in such things, or has the technical knowledge to indulge in this (I could be wrong - if so I appologize). Overclocking not only requires significantly more elaoborate (read expensive) cooling systems, it VOIDS the warranty on the chips that are overclocked. This is a point to consider. There is a most interesting and useful post in the sound card forum re the impact of stand-alone sound cards vs on-board sound in Tom's Forums - you may want to check it out.
On the issue of Windows Vista vs XP, that is irrelevant. Vista is still in (relatively early) Beta testing, and therfore won't be out for at least 6 months. More like a year, so don't worry about it. Furthermore this is just one more reason to wait a few more months before buying a new system.
A web site that you should seriously consider visiting is www.annoyances.org This a great site for all kinds of info and solutions to "quirks", "undocumented features" and short-comings of MS operating systems and software.
Hope this helps.