Hi Nora,
I'm just starting out with editing but have been reading quite a lot on the subject. only having 2 drives in raid will give a big performance boost but is also very unsafe. If one drive crashes you will loose all your data. Better would be to invest in an extra single drive for your applications and then your 2 drives in raid for capturing. 500Meg seems quite a lot but depends if you want to save more then 1 project on it. You might save a lot taking only 2 x 120gig in raid0. With the money you save you could buy an extra WD raptor disk for the applications.

If you work with premiere pro a simple firewire card will do, capturecards have the advantage to work with analog as well but they come in very different priceclasses. From what I heared from the real pro's is that they suggest to first buy your capturecard and then build your system around it. I planned to this also but it was way out of my budget. For that reason I bought a pc with a firewirecard. I guess that if you can afford it you should buy a good capture card, on the adobe site there is a list of certified third party capture cards -http://www.adobe.com/products/premiere/6cards.html so that you are sure that premiere will have no problem with it.