Well, the rig you've outlined above certainly seems like it'll overclock just fine for you. However, achieving 2.5GHz on air, even with a new low-volts XP, might be a bit of a task. I imagine 2.2-2.3GHz on air would be a reasonable assumption with this chip and setup, but more may require additional voltage, adding to heat. Make sure whatever case you pick has plenty of room for added fans. Also make sure that whatever case you end up picking out meets the space requirements for all the drives you want, and the rest is just a matter of personal tastes and expense. Keep in mind that you usually get what you pay for, and cheap cases usually come at some expense, usually the PSU. You're going to want to make sure whatever case you get, that the power needs for this system are going to be met. Not all PSU's are built the same, and some PSU's burn out before they even get near their rated wattage.
Keeping in mind that the A64's and FX's both have access to additional instruction sets, this should keep you pretty much above the game for at least a couple years. You should be able to find a 9800 Pro 128MB card pretty cheap right now, especially OEM.
As far as cooling goes, without going into extremes with water cooling, I'm not particularly sure. The last cooler I installed on a socket A system was a Vantec Aeroflow, and that cooler worked just fine. You might want something with a bit more cooling capacity for overclocking sake, but either way this link might be a bit helpful in making your decision. Not complete, but alot of tests conducted over here.
http://www.overclockers.com/articles373/socketA.asp
EDIT: One more very important detail: Assuming the rest of the rig can handle it, a cheaply made power supply can definately affect your overclock. If it's unable to provide stable voltages under extreme situations, you'll start getting random hardware errors and reboots. If you invest a few more dollars into a good quality PSU right now, you'll definately get as much as possible out of the rest of your system.
That should help you get started, and welcome to TechIMO!
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