For what my two pence is worth, Not if you bought an awesome graphics card ages ago and are picking up a duplicate at a knock down price. Never go straight to SLI. Buy the best u can afford as a single card and then later when u want a performance boost your original card should be cheaper so u can pick it up and get a better cost effective performance boost than shelling out on a whole new single awesome card.
Can't help you with your PCI X probs, as long as both cards are in a x16 slot they will work, boards with two x 16s are generally more expensive. Make sure you have the right chipset for your card. ATI cards need cross fire and Nvidia cards need their nForce.
I am getting an 8800gtx, single card. The sli will happen down the line. I just need to know that I am getting the proper motherboard for this.
Just make sure the north bridge is the nForce 680i (or nforce 590 if you've got an AMD processor) and you'll be fine. From your original post, the motherboard you're looking at has to be based on one of these.