Here's a few suggestions. Take 'em for what they are worth.
First off, if a 'techie' works for BestBuy, forget what he/she/it says...
Anyway, as for a mobo, consider this one:
DFI nF4-DAGF Socket 939 NVIDIA nForce4 ATX AMD Motherboard
Not bad for $89. I just read a great review of it, Here:
It really sounds like a solid board. I am considering it myself.
As for a case... Are you considering the Super Lanboy because you can get a discount? You do know it doesn't come with a PSU?
Go with Antec (great quality). If you don't have to have a side window, check out this one:
Antec Performance II SOHO File Server - Black
(BestBuy link, so you can get your discount)
Antec PERFORMANCE TX TX1050B Black Steel Server Computer Case 500W ATX12V v2.0 Power Supply - Retail
NewEgg link to a better version, same type of case.
If you have to have a window, check out these two ThermalTake cases, both with PSUs:
Thermaltake Soprano VB1430BWS Black Steel/Plastic ATX Mid Tower Computer Case 430W Power Supply - Retail
Thermaltake SOPRANO VB1430SWS Silver Steel/Plastic ATX Mid Tower Computer Case 430W Power Supply - Retail
As for a CPU, the mobo I suggested is a 939. Go with a Venice core. Buy what you can afford, but don't worry... that mobo can overclock with the best of 'em. You may want to get a decent after-market cooler first, though.
A 120 gig HD ain't bad, but for the prices, you can go better. Use a 120GB SATA drive as your OS drive is good, with about a 200gig data drive for your music, files, movies, whatever. That way, if something goes nuts and you have to format and reinstall, you don't lose your data.
RAM: I'm no expert on OC RAM, but PC4200 is a bit much. I don't think AMDs recognize the difference past PC3200 (if I'm wrong, I'm sure someone will tell me). Using a gig is a good idea, though.
Go for a DVD burner (double layer) as well as a DVD/CD-RW for a second driver. DVD burners have come way down, to the tune of roughly $40 or so for an older model. Check NewEgg for the best selection. You will never regret having a second drive, trust me.
For your vid card, get what you can afford. The newest and best is great, but it is expensive. Just be sure to go with a good company. Read through some of the threads discussing this topic and you'll see which are good, and which aren't.