kargooliw, if you install two NICs, even with a huge pipe to the net, you will be able to setup full duplex; meaning one card for incoming traffic, and one for outgoing. This will greatly reduce the bottleneck between your cpu and the world. Even if you're not going to experience a lot of hits or traffic on your site, NICs are cheap enough to justify the rewards.
As for one cpu vs. multiple, a team of horses pulling a wagon is going to be more efficient then one monster breed steroided out. I have a multi-processor server, running P-II 400s. A few of those (which you can almost dig out of someone's trash) will out perform any single processor 1.x or 2.x GHz machine. The initial expense if higher, as you are talking about server hardware, instead of a modified desktop, but check e-bay, ubid, overstock, etc. They have surprising deals on Compaq servers! I will still be using my P-II server long after others have upgraded their single processor box. I don't know why you would want a custom video card on a web server, seems kind of silly to me. A cheapie would work, as your really don't want to run anything over 256 colors anyway, it is directly proportional to needless resource, and budget usage.
If you are just doing user authentication, you will defiantly want to go with a full duplex connection, and a min of a gig of ram. Drive space isn't too critical, but make sure it's mirrored, and you do backups! If you are doing databaseing or similar drive dependency work, then HW RAID-5 is an exceptional solution. It's expensive but once again you can pick up a controller card aligned with the price range you are targeting, drives are also a little expensive, but from an operational and disaster recovery standpoint, if you can flip the bill, well worth the cost-trust me.