A separate graphics card is a must...onboard graphics borrow a LOT from the system memory....bad juju.

As for the RAM: Depends on the mobo you choose and its RAM compatibility (DDR2 or DDR3). DDR2 memory will do you just as fine as DDR3...and be less expensive. DDR3 is the latest version however and a tad faster (generally speaking...depending of course on individual speeds...a high-speed DDR2 will beat out a low speed DDR3). But if you have the bucks, go for broke and get a DDR3 compatible mobo and spend on the DDR3 RAM. Corsair, OCZ and Kingston are all excellent brands. For your intentions you really don't need to get the high-speed variety either. 4GB of DDR3 1600 can be had for around $80...you can always upgrade later (to the mobo's specified upper limit).

Now that I have muddied the waters....