Ok, here's the problem with your current thinking. You only mentioned first person shooters, which are usually the game genre that takes advantage of new technology. A cabinet is not good for first person shooters, especially todays. There is nothing new, or going to be new that is cabinet worthy that will use directx 9 for a couple of years. Those games require a seperate computer, keyboard, and mouse

For a cabinet the arcadevga will be more than enough. Actually, if the arcadevga came in 3dfx then it would be great for all the emulators/pc ports of arcade games out there right now

Get a radeon 9800 for your normal PC and play first person shooters on that. There's a reason you don't see too many 1st person shooters in the arcade....
If I put a voodoo1 in my computer versus a Geforce2 and play quake3, don't you think i'm going to see a speed and overall quality increase--even at 800x600?
Well, yeah, with those cards. Your talking the start of decent 3D cards versus newer refined ones, when 800x600 was considered big. With today's cards you aren't going to see an increase in the 800x600 performance. It's performance will be only look one way at that res: FAST!
I'm just saying for the little bitty bit of difference you will see between the arcadevga and a card that supports dx9 is VERY little and not worth the effort of trying to have both cards in an arcade machine. It's like buying c4 to shatter normal glass. It's overkill for the application.
Actually, rereading it sounds like you only have one computer right now? I suppose, for the budget. However, there isn't anything now that takes full advantage of dx9. So until that happens, save up about $200 and build a dedicated arcade pc:)