Yo -- "First-Time"!!
Good question. Already -- some good answers.
You have to decide what you want, but I see you're of the practical mindset, and worried about cooling.
My friends led me astray a year ago. In fact, not only my friends, but Maximum PC Magazine -- for using the CoolerMaster WaveMaster in building the "Dream Machine of the Year (2003)".
I seldom resell my machines, preferring to keep them through the 10-year CPU-life-expectancy as drones for proxy-servers, file servers and other uses.
Last fall, after purchasing the CMWM and thinking that it was "cool enough", I retired a 1995 full-tower Gateway 2000 Pentium 166 system. The case is solid steel, and roomy; the bezel was a very solidly designed, conservatively stylish affair. The case design was somewhere in the no-man's land between AT and ATX, and I discovered that very little had to be done to the motherboard pan and I/O-port rectangular cutout to make the case "ATX" friendly.
My cooling solution puts two 120mm fans, one on top of the other, below the power-switch and LED's in the middle of the bezel. To save room inside the case for hard-disk cages, I bolted the fans on the outside of the front-chassis panel -- between the chassis and the bezel. In the case of the Gateway full-tower, the fans just fit perfectly behind the plastic bezel ventilation grill, with room for some aluminum fan filters.
The exhaust fans consist of two 92mm variable speed "smart" fans just beneath the power supply. I had to cut holes for these with a nibbler or Dremel cutoff tool. The only problem with putting the 92mm fans just below the PSU is that they have to run at a certain speed, or they starve the air intake for the PSU's internal 92mm fan which sits perpendicular to the direction of flow for the exhaust fans. Probably something like a "Bernoulli effect", but I found that the intake, exhaust and PSU fan speeds could be varied for good balance. Finally, the CPU fan is a 120mm LED Smart fan using an acrylic 120-to-92mm adapter, which in turn bolts onto a PIPE101 heatpipe cooler. I strategically located a blow-hole duct in the case side panel and equipped it with a vinyl-plastic blow-hole duct for 120mm fans -- purchased at www.frozencpu.com.
I stumbled across your post here, initially intending to respond to today's (Sept 13, '04) article on mid-tower cases. Mid-tower cases do not seem to have a cooling profile that is superior to full-tower cases if the latter are equipped with thoughtfully chosen fans and ducts, and the articles always seem to ignore any quantitative measure of "case-cooling-effectiveness."
My most recent project jettisons the CoolerMaster for a system based on the ASUS P4T533R with proprietary 32-bit Rambus. The 3.06B processor is just a tad hot. I was lucky to find a (free!!) InWin full-tower case not too different from the Gateway described above, and implemented a similar cooling solution. (Except that the exhaust fans sit above the PSU and do not interfere with the PSU's air-supply). The InWin was probably circa 1998, fully ATX with removable motherboard pan. There is not enough room between the front-chassis-panel and the bezel for the fans, so I made a rectangular cutout for two 120mm fans as before, and fitted it with a "modders' mesh" perforated steel extrusion that resembles a stereo speaker grille.
For between $100 and $200, you can buy some cases that facilitate cooling, have stylish looks, and are advantageously equipped with front and rear-panel 120mm fan ports. I'm thinking of a recent Lian-Li entry which actually puts the PSU at the bottom of the case, sucking air off your carpet (it has wheels, too, so there is space for this intake ventilation), and also, the CoolerMaster Stacker. For cooling, the Stacker is ATX-to-BTX convertible and is a far cry from the WaveMaster's cooling shortcomings. If I were self-obsessed, I'd think that CoolerMaster stole my design.
The Lian Li -- forgot the model number/name -- is also convertible, I think, but you might investigate further.
But -- Hey! -- free is free! Extra pocket-change for ME!