Definitely, if you already have the parts. You should be making >$2/day after electricity costs right now. (keep in mind, advertised $/kWh rate is NOT true kWh rate after fees and taxes, which isn't a figure most electricity providers furnish. You should divide your total bill by kWh consumed for a better picture if you're just relying on the numbers in the bill)
To get a "real" idea, buy a Kill-A-Watt (or similar) if you don't already have one. Measure power consumption, plug numbers in @
http://www.bitcoinx.com/profit/ . If you keep the computer on while you're not using it, make sure you account for this by subtracting power draw while hashing from power draw while idle, since that'd measure your real cost over what you're currently doing.
You should also keep in mind that if your temps are fairly high, this will cause the GPUs to "degrade." Heat's bad whether hashing, gaming, or anything else resource-intensive. AFAIK, there are no good studies on how running a GPU 24/7 under various temperatures affects MTBF (I'd like to be proven wrong, though!).
