Right now, I would bet that you couldn't tell a $100 difference in gaming performance, but maybe that's just me. You could easily overclock the 3800 to 4200 speeds, on stock cooling no problem, to make up for this difference.
As far as RAM goes, you can get faster memory (up to PC4400 DDR550 is the fastest I know of), however in order to get it to run at these speeds you will have to overclock your system.
This is really personal preference, however I would make sure to get one that has an SECC steel chassis. Otherwise you may end up with a case that feels like it is made of tin foil. Just find one that you like the look of and make sure that it is either SECC steel or aluminum. A full tower will be HUGE, so I would recommend a mid-tower.
Again, personal preference on whether or not you want fancy lights/etc., however, I would make sure the case has a 120mm port on the back, and either 1x120mm or 2x80mm up front, as well as 1x80mm on the side (optional, though it will help temps a bit). Good noise levels are below the 40-45dB range in my opinion, and good airflow varies by size. A good 80mm fan will move at least 30-35cfm, while I would probably try to get a 120mm fan for the rear that moves at least 60-70cfm. Newegg
has a very informational system that will tell you all about the fans you are looking at. You can even search by air flow or noise level from the advanced search!
Hope all this helps.
AMD Opteron 165 CCBBE (2.7GHz @ 1.35v)
DFI CFX3200-DR (07/28)
Kingston HyperX DDR500 (3-3-3-8-1T)
Asus Dark Knight Radeon 4870 1GB
2x 74GB Western Digital Raptors in RAID0
500GB SATA-300 WD Drive
CoolerMaster RealPowerPro 850W
NZXT Zero full-tower case
Windows XP Pro SP2