Firstly gaming wise a P-D will make very little if any difference in performance...getting a lower clocked P-D than your P4 will cause frame rate to be lower, you have to realise that a P-D is just 2 P4's on a single die...so clock for clock in single threaded applications(IE 99% of games) a 3ghz P-D is identicle to a 3ghz P4.
For me the cons of getting a P-D far out weigh any benifits you would get from the second core....sure some multi threaded application and generally multitasking will be slightly improved but the high temps can lead to a few problems(like overheating and clock throttling), especially if the ventilation in your case is very ordinary.
Secondly as I have already said and others have too, SLI is a huge wank..epsicially with a lower performing CPU like a P-D...and two video cards, especially higher end models can generate considerable amount of heat, which the could make an already overheating P-D problem worse...not to mention the beefy PSU requirered for SLI.
and last but not least...ewwww ECS....but you've already heard what I have to say about that.
If I was you and this is just a suggestion, I'm only trying to but you on the right path, I would get a could 965P motherboard, the Gigabyte one seems very popular...but really ask Duvie and Stevty2889 about a good motherboard they are more than willing to help you out, and just maybe get a decent HSF unit and overclock your P4...you should be able to get a decent boost out of it, probably more than what a stock P-D could do for your gaming.
And then save up a few pennies down the road and buy a C2D and a GF8800 and live happily ever after.
this is just my opinion.