usually any motherboard that has a form factor above mini or micro atx (atx, extended atx, super atx) will have atleast 4 if not more expansion slots available by default and normally an atx (and possibly a extended atx) motherboard should fit in a full tower case (a super atx motherboard will more than likely need a super tower)
microBTX
A pico BTX is 10.5"x8" and only supports one or two expansions slots, designed for half-height or riser-card applications. A microBTX is 10.4"x10.5" (264x267mm) and it supports up to four expansion slots.
Four is a good number for most people.That gives you a lot of flexibility.
Yes. The ASUS K8U-X has four PCI slots.
The Intel P45SG can support up to eight gigabytes of memory across its four memory slots.
The Asus P4SD can support up to four gigabytes of memory with its four memory slots.
Beleive it or not, this was the original definition of a 'desktop'. Before that it was differentiated as a 'tower' for systems you did NOT set the monitor on. The form factor was typically LBA, as opposed to things like AT, ATX, etc.
It is a technology that theoretically doubles data throughput from the memory to the memory controller. The motherboard has memory slots and if not using all four slots, then they must be placed in a certain configuration for it to use dual channels. Your motherboard manual has the configuration of memory and slot placement for this to work. If running dual, it is best to buy memory that is designed as a pair or a set to get the most efficiency, it usually says, dual channel kit.
DDR3, you can check with the manufacturer if you're not sure.
If you install DIMMS in all four slots that don't match, the memory will still work, just not at top performance.
This would depend on your Operating System and motherboard. For example, if your computer runs on 64-bit Vista with 4 DIMM slots, you could upgrade up to 8GB with four 2GB sticks.
If your SATA controller can handle four drives (if it has four SATA ports, then presumably it can), then yes, you can use four drives.