You have 2 options at this point in SHA mining, USB ASIC things like the AntMiner ( since Erupters are getting really rare ) or Video cards.
With the USB option you need powered hubs and assume that the 500 mA per USB port on a PC is what each unit will use. Each AntMiner can get ( with out fooling around too much in hardware or software ) close to 1.55 GHash/sec you will want to consider a minimum of 3. That could get you 0.01 BTC every 12-20 days at current difficulty ( based on my earnings at BTCGuild and it's estimated 24 Hour Earnings 0.00046300 - 0.0005872 ). The other advantage for USB is that you can use a PI for the PC that does the running of CGMiner or BFGMiner to run the USB devices and have the total power from wall plugs down alot compared to GPU Mining. The disadvantage is that AntMiner ( and similar, with only DualMiner as the current exception ) can NOT do any script mining.
The Video Card mining option is based on the number of PCIx risers you can plug in to EACH mother board and how many power supplies you can chain per unit to power the cards and the ' luck ' that the board can actually USE the cards. There are still PCIx16 motherboards that follow the 1.0a standard that will refuse to see video cards that follow the 2.0 standard and a few of the 2.1 ( or higher ) standard cards. To restate that last one, the standard for the actual PCIx16 slot has had an issue where motherboards went to the 1.0a standard and alot of the video card manufacturers that waited until the 2.0 standard came out made the cards not compatable with the 1.0a standard and thus can't be plugged in and used. You also need to have enough 12V amps available from the power supplie(s) for the cards and motherboard PER RIG. The advantage of OPENCL compatable cards is that they can do almost any Script mining OR SHA mining ( there are a few coins that use an altered script that needs to be compiled on a LINUX miner in order to work and the directions don't always work ). The disadvantage of GPU mining is the shear power costs. You can see your power bill go up over $100.00 per month if you get enough GPUs running ( yes, over estimate to bring home the point of how much wattage is needed ).
PROs, Cons, options and budget are all part of deciding. Hit eBay and search for current video cards and how much they are going for. I recomend staying away from ATI HD 4850 as It took me a week to get the drivers to load in Ubuntu AND have it mine, if I could have afforded a HD 5850 it would have been closer to plug and play. On the Nvidia side of GPUs, the lowest card I have ( mining ) is still a very well used 8800GT that has gotten a heat sink kit ( copper shim and new paste, fan bearing grease ) and instantly reached what hardware comparison pages say that it would mine.
https://litecoin.info/Mining_Hardware_Comparisonhttps://en.bitcoin.it/wiki/Mining_hardware_comparisonIn the end, how much have you got to spend for the setup and what hashing power do you think you want/need is the question you need to ask yourself.
Hope this rant is helpful.