To really answer this question, you should tell us what you plan on playing in this cab. If all you're playing is classics, you're overspending and wasting money.
If you want to play as many newer games as you reasonably can, or are using the cab for other high end emus like Dreamcast or PSX, or for higher end PC games, then that changes the entire equation.
Your PC specs look fine to play almost all of the games in MAME. You'll be able to play some CHDs and most newer stuff will run well (out of the games that can run well at all). Jumping to a gig of ram might help on some of the larger roms, but if this is strictly a Mame box, you probably won't see a big difference.
Dual core isn't a big deal with Mame as mentioned. Clock speed is the more pressing factor, so getting a faster CPU may help you, but it won't help you much BECAUSE its a dual core... it'll just be since the chip is faster.
Video card is again, highly dependent on what you are doing. A classics only machine can probably run on any damn video card that you can find that works, including one that you pillaged from an old machine. For newer games, PC games, other emus etc, something with 128mb ram is probably a safe bet... again depending on what you plan to play.