PC2700 and DDR333 are the same thing. It's just a different way to say the speed. You could install faster RAM (the next step up is DDR400/PC3200) but it would run at the speed of your slowest RAM so the increase in price would be useless. DDR is all 184-pin (not DDR2 - that's 240-pin.) ECC can be taken by any computer but it's expensive. Pairs of ram CAN speed the computer up if it runs in dual-channel but you probably wouldn't notice. You can install any brand of RAM but some perform better than others.
I would reccomend this:
http://www.newegg.com/Product/Produc...82E16820134008
It's Kingston (a fairly reputable brand), a single stick in case you want to upgrade later, has good timings, and is DDR400, which is the newest standard for DDR.