It depends on what you'll be doing with your new PC as to whether SLI or Crossfire is worth it for you, either one is an expensive route to take, 2 video cards is twice the money, an SLI or Crossfire M/B cost more, so for you the question is what will you be doing with your PC?
If surfing the web, emailing your buddies, and watching DVDs, and occasional low end gaming like online Pool or Card Games is your thing then you don't need either of the technologies.
If you're a serious gamer wanting to crank up the resolutions to max settings and play wide open through your game, and into HiDef DVD playback, then SLI and Crossfire may be up your alley if you're willing to spend the money to get there, but like I said it depends on what you do with your machine.
Now even in gaming just because you have for example SLI, if the games are a little older and not written or patched to take advantage of it, some won't even run in SLI mode, in some games I've had to shutdown SLI, but the games that run good on it, are worth the added expense to me, and luckily for me the games I love the most run fantastic in SLI Mode.
I'm trying to be straight forward here and I don't want to belittle what SLI can do in games that are software optimized for it, for example the game FarCry when it first came on the scene was kicking good systems of the time in the dirt.
I first ran the game on an ATI 9800Pro, it limped through the game and occasionally crashed, then moved to an ATI X850Pro the crashing disappeared and settings were increased, but SLI gave me the advantage to completely Max Out All the video settings and literally run through the game, so SLI can really give an impressive graphics boost, in some games.
Obviously by now you've figured out I'm running an SLI setup, so some other points I'd like to make is with HIDef playback on the computer, I've noticed smoother playback with SLI enabled, however I'm running Power DVD 7 and its optimized for SLI use, I have a 27" HiDef LCD TV/MONITOR that I watch DVDs on and enabling and disabling SLI does affect the smoothness and crispness of the picture.
Benchmarks really show the difference in a Single and SLI setup as far as the raw performance is concerned, and 3D mark scores give you more of an idea of the gains.
My Benchmarks
Single 7800GTX
3D Mark 01 = 33,153
3D Mark 03 = 21,486
3D Mark 05 = 9,586
3D Mark 06 = 5,013
Dual 7800GTX in SLI
3D Mark 01 = 34,221
3D Mark 03 = 34,521
3D Mark 05 = 13,738
3D Mark 06 = 7,804
The benchmark in SLI of 3D mark 01 is a good example of the older software really not optimized to take advantage of the SLI, as is some of the older games.
I don't know if this is the kind of information you're really looking for or not but for myself I would like for SLI to have been totally killer across the board, at least for myself it would justify all the added expense, SLI is really an occasional serious boost to what you're doing and sometimes a single card will get you further, it depends on what you're doing at the time.
Keep in mind all of todays and future software coming out will be optimized for SLI and Crossfire use so things are just going to get better and better with both technologies.
I hope this helps answer some of your questions.