I went with the Antec Nine Hundred case instead of the P180 because it had more cooling (bigger fans) and seemed more sturdy. I got mine for about $115, but I saw them go as low as $100 at Frys.com, so you would have to check around for the best price. Newegg has it for $130 (if your not into doing rebates), but check on pricewatch.com for a different vendor with a cheaper price if you are interested. There is always someone who has a slightly cheaper price, just make sure they are reliable.
Newegg.com - Antec Nine Hundred Black Steel ATX Mid Tower Computer Case - Retail
As Grim mentioned, you should go with the DDR2 ram. The value ram of PC9200 was about $200 last time I checked at Newegg.com (its now about $215)
Newegg.com - OCZ Reaper HPC Edition 2GB (2 x 1GB) 240-Pin DDR2 SDRAM DDR2 1150 (PC2 9200) Dual Channel Kit Desktop Memory - Retail
Here is a couple cheaper options:
XMS
Newegg.com - CORSAIR XMS2 2GB (2 x 1GB) 240-Pin DDR2 SDRAM DDR2 800 (PC2 6400) Dual Channel Kit Desktop Memory - Retail
Standard Crucial:
Newegg.com - Crucial Ballistix 2GB (2 x 1GB) 240-Pin DDR2 SDRAM DDR2 800 (PC2 6400) Dual Channel Kit Desktop Memory - Retail
The E6600 quad core also had been coming down a bit lately if your interested in that as well.
Newegg.com - Intel Core 2 Quad Q6600 Kentsfield 2.4GHz 2 x 4MB L2 Cache LGA 775 Processor - Retail
This also means that the dual cores are going to be cheaper, so it really depends on how much you want to spend.
When you get a power supply, make sure it's a reliable brand (Antec, Coolermaster, Thermaltake, etc.). I have had a few default ones (came with old case) that blew out after about 4 months. Also, look for one with a high power efficiency (around 80 percent), it will last longer usually. At least that is what all the experts say. My PSU (85% efficiency) has been running strong for a year now without even a hint of a problem.