A power supply is rated for the maximum power (in watts) it can output. A cheap power supply might be rated for a maximum of 300W, but in reality output more like 250W average. On the other hand, a really good 275W power supply might output 275W almost all of the time. In this example, the name-brand 275W power supply is much better than the non-name 300W power supply. This is why you can power an AMD Thunderbird with 250W and 275W power supplies.
I've got a 10K RPM Quantum SCSI hard drive, a Plextor SCSI CDROM, a Ricoh SCSI CDRW, dual Pentium IIIs, an old Maxtor EIDE hard drive, four PCI slots filled, an AGP video card, and 2 extra case fans (plugged into the power supply).... all on a 250W power supply. It runs flawlessly under both Linux and Win2k Advanced Server.
I can guarantee you that my 250W power supply supplies a lot more power -- on average -- than most of your 300W power supplies.
Try to buy name-brand cases and power supplies, if you can. Inwin and Enlight are two very good brands (ones that I've stuck to for over a decade), but over-clockers might prefer something with better cooling options. Supermicro's SC750A and 760A are both good server cases that could keep a dual Xeon cool. They run around $100+, while Inwin and Enlight server cases can be had for $80+.
Go to
3dfxCool for case fans and HSFs. They are the best. If you live in the midwest (Dayton, Ohio area), you might want to stop in at
5 O'Clock Computers, as they stock some good cases and have excellent prices for walk-in customers (unlike every other store within 200 miles of me, and I
have checked most of them). Mail-order prices aren't major bargains, but they're good enough to compete on pricewatch. Service is abysmal and the owner is annoying, but you only have to talk to them for 2 minutes. I like their prices enough to tolerate their attitude problem.
