My First Custom Machine

I’m a complete hardware noob, but I’m trying trying to build my own computer, for the first time.

I intend to scrape these components from my current Dell machine:
[]Nvidia GeForce FX 5200
]512 and 256 mb PC3200 (DDR400)[/LIST]
So, here’s what I’m looking at:
[]Case: Antec Performance I P180B Black 0.8mm cold rolled steel for durability through the majority of chassis 1.0mm cold rolled steel around the 4 x HDD area ATX Mid Tower Computer Case - Retail. $109.99. (I think I could save money by getting a cheaper case.)
]Motherboard: DFI Lanparty UT CFX3200-DR Socket 939 ATI CrossFire Radeon Xpress 3200 ATX AMD Motherboard - Retail. $89.99. (Really great deal, today only.)
[]PSU: FSP Group (Fortron Source) FSP400-60THN-R ATX 400W Power Supply - Retail. $66.99.
]Processor: AMD Athlon 64 X2 4600+ Manchester 2.4GHz Socket 939 Processor Model ADA4600BVBOX - Retail. $247.99. (939 Sockets isn’t so bad, is it?)
[*]Memory: Kingston ValueRAM 512MB 184-Pin DDR SDRAM DDR 400 (PC 3200) Desktop Memory Model KVR400X64C3A/512 - Retail. $55.99. (Remember, already have 768 mb.)[/LIST]
$570, so far.

So, what am I forgetting? What do I have no idea what I’m doing about?