Is it worth buying DDR3 now and if not, then can I buy a DDR3 motherboard and use DDR2 memory for now?
No and no. DDR3 is still to expensive for what it offers, all current motherboards won't be able to properly support DDR3 when it reaches it's prime, the clock rates should dramatically increase as the technology matures.
How much of a difference is memory at 800mhz and at 1066mhz
Performance wise, virtually none. If you are overclocking with anything but a nVidia chipset, you'll need the higher frequency for additional headroom.
Memory....corsair vs patriot?
That can't be answered without a budget, also I wouldn't limit yourself to those two manufacturers.
680i or 780i
That depends on your budget, do you plan on overclocking? Do you plan on running SLI?
I'm wanting to run 4GB of RAM. Looking at running XP Pro for now. Will 4GB run on Vista in the future (I hear it has issues now)
There is nothing wrong with Vista, I'm on it right now and havn't had any issues with it yet. If you choose an nVidia chipset and you're dead set on 4GB of RAM you'll need a 64bit OS to support all 4GB. A 32bit OS would only see about 2.75-3GB of RAM, assuming you had a high end video card as well.
Is it worth the money to buy something like the KI Gaming Interface Network Card?
Intel has a very cheap networking hard for 30$, it's just as good as the killer card, but at a fraction of the cost. Is it worth it? If you download stuff it is (from what I hear) and if you are typically far away from the servers you play games on (you'd notice this if you had pings higher then 100ms).
Is Intel a good motherboard...or should I go with EVGA
Intel doesn't produce any nVidia chipsets, again, do you wish to purchase SLI? I highly recommend you don't purchase SLI unless you are running a 24" monitor or larger and have money to burn.
Q6600 or E6850 ?
Neither, I suggest the Q9450, it'll be out very shortly, it's already appearing on the market.
Is it a plus in performance/compatability to buy a video card and motherboard by the same maker.....say EVGA?
Many people believe this, but there is none.