I'm interested in getting involved with bitcoin, but all the different methods of mining is a bit confusing. Is there wild performance differences amongst the different implementations? Is it necessary to stay on top of the latest builds and whatnot?
There are two basic ways to mine bitcoins. You can either use a CPU miner, which does all the hashing calculations using your CPU, or the GPU miner which does all of the hashing calculations on your graphics card. There are other experiments in mining (for instance, there's a guy working on hashing with an Field Programmable Gate Array
which could probably yield some serious hashing capacity), but generally speaking, though, unless you've got some custom hardware lying around, those are your two main methods. The GPU method is preferred because the processors in graphics cards tend to be better suited to doing this kind of work, can do a lot more of it in a massively parallel way (there are more arithmetic units in your average GPU than in your CPU, by a huge factor), and it doesn't slow down the rest of your system in much the same way a CPU miner does, since the whole rest of your computer still needs the CPU to do all of its work (though the GPU tends to idle most of the time you're not playing 3D games or playing HD video).
I have an i7 with a HD5850 and a C2D iMac to mine with. I assume I could use both rigs to mine with.
What app/software should I be using? Do I run the standard bitcoin app while running the miner apps? Do I need separate wallets for the 2 rigs, or even separate workers?
You could run the bitcoin app on both machines, but then you would be maintaining two separate wallets. Nothing particularly wrong with that, you could just throw all the coins from one wallet into the other. Preferably you would run one of the GPU calculators on both of the machines, to optimize the amount of work your machines are doing. I prefer DiabloMiner, but there are others floating around out there. Sorry, but I'm too much of a newbie at this myself to give you too much advice there.
You could also join a bitcoin mining pool. Check the wiki for an explanation about the different pools and their policies, but basically you join in a big gang to chew through blocks quickly, and you get a portion of the coins mined. What you can do there is set up one wallet (or use an online eWallet, like MyBitcoin), and have your portions of the profits sent there. The payouts are less profitable, but they tend to be more regular, and you have a higher probability of making a few coins than if you went it on your own. For example, my MacBook Pro is running about about 4 Megahashes per second, and theoretically I would generate one block every 1,800 days (unless the difficulty factor goes up). In this case, I feel it's more advantageous for me to join in a pool because even though I'm technically making less money than I would on my own, I don't have to wait potentially five years to see any of it.
Is there a newbie guide anywhere that goes over basic details of how it all works?
I wish. Someone really ought to get on that. I might, but my work has me pretty busy these days.