Well, i was currently looking at building one myself recently, and i have a decent build that i can send you at 900 bucks (not counting shipping and handling, unfortunately) that came at 2.4 GHz quad core (easily over-clocked to 3.0), 2gb DDR2 ram, and a geforce 8800 GT card (256mb though), along with a fairly upgradeable MOBO. let me know if you want me to send you it and see if it works for you.
as for your questions -
DDR2 and DDR3 - the difference seems to be that one can take in more data, but returns it slower. Meaning to get the best use out of DDR3 you need a fairly large data going into it - which explains why the highest DDR2 is roughly 1060 or 1300 MB, but DDR3 currently goes up to 2000 or so. but basically, it won't make a significantly difference. lookie this thread if you want to read it for yourself, since i probably did a crap job at summarizing it - i don't get it very well myself.
http://www.tomshardware.com/forum/249591-30-ddr3-ddr2-ddr2
As for power consumption, a friend of mines recently showed me this site:
http://extreme.outervision.com/psucalculatorlite.jsp
i haven't used it yet, but it should give a fairly good estimate of your set-up's power consumption. basically - if you plan to upgrade, try going 50-100watts over what they give you.
And of course, when buying pieces, i recommend Newegg.com. Decent prices, easy to navigate after a while, etc., etc. it just kicks ass.
Hope that helps =]
EDIT: I see you already know about Newegg, goodie. just a note - on the side bar when looking at power sources, there will be "guided search", "power search", and "Advanced search". If you click on Advanced seach you can specify components, such as "PCI Express 2.0 Slots", and select how many you want.
just a side note - i've been really fond of this one:
http://www.newegg.com/product/product.aspx?item=N82E16813131284
Good FSB, 2 PCI 2.0 slots, 4 RAM slots and supports DDR2, and a bunch of other crap. enjoy
um..stuff
