We're just getting to a point where quad core and higher processors are becoming normal. Most games to this point have been programmed to run on dual core processors. Now that enthusiast gaming computers are almost exclusively using quad core and eight core processors are coming out, game designers will probably start incorporating more cores. That is all purely conjecture, though. No one can accurately predict what games will look like three years from now.
The price differences between motherboards are the features but there are more important things to pick first. First there are a few sizes available and some motherboards need to be tiny to fit into tiny cases. Those cases aren't as linear in terms of price vs. features so lets assume we're talking about ATX boards (this is easily the most common size). The first differences are the socket. The i5-2500K uses an LGA 1155 socket whereas some older intel processors might use LGA 1366 or others. Socekt type doesn't really affect price but it's the first thing that you need to check. The second thing is chipsets (or northbridge). For the i5-2500K the common chipsets are H61/H67, P67, and Z68 in order of increasing price. Higher prices correspond to newer chipsets and better features and for the 2500K you'll almost certainly want Z68.
Once you pick your socket and chipset, you've got a long range of prices (from $100 to maybe $375 for Z68). One of the differences is the port cluster. More expensive boards have better features like more usb 3.0, dual ethernet, 6+ audio ports, eSATA, and others. The next (and arguably biggest) difference is in the expansion card slots and specifically the PCIe 2.0 x16 slots. The higher cards have more PCIe 2.0 x16 slots and the pros and cons of those can be discussed in a different thread.
There are extra little tidbits to look for like PCIe 3.0 x16 slots which are the newest generation (which will not be supported on sandy bridge but will be supported on ivy bridge), usb 3.0 headers on the motherboard (as opposed to the cluster), features like turbo boost and uefi bios (although most have uefi these days), and of course heatsink effectiveness and overall look.