I know just enough about computers to be dangerous
Well, software-wise I'm competent. Hardware I know more than the average joe, but not enough to know the answers to this.
And I know there's people out there that are very computer-savvy, so I'm asking away!

I have a Compaq Presario (my family has had good luck with HP so far, plus it was on sale) with an AMD Athlon X2 5400 Dual-Core processor, 3G Ram (Hmm, I saw 4 Ram cards in my computer though, I just opened it. Didn't look at them in detail, 2 must be 512s or something), and an NVIDIA GeForce 6150 SE Graphics... thingy. (Running Vista 32bit if you must know; no issues with it though.)

I discovered that my Graphics is integrated, and I've an empty PCIEx16 slot, which is used for graphics cards I'm fairly certain.

So here's my questions:
1) If I were to get a better, actual graphics card and put it in, how would it interact with the existing integrated graphics?

2) Is a better graphics card the thing to get to increase the settings I can play games at? I can run DDO at Medium-ish settings; I tried playing DragonAge and it runs very slow and choppy on the lowest settings, barely playable (I did become a Grey Warden before I gave up on it though ). If so, what would you recommend, while staying cheap?

3) If a better graphics card is NOT the answer, or if it's not enough, what else can I do/get to increase my computer's performance?

Thanks guys.