Adding a HDD?

My 250gb Western Digital conked out on me a few weeks ago and so i used my old 80gig Seagate for the time being.
Now i have a new 250Gb WD sent to me as a replacement can I just plug it in while my seagate is hooked up and use 2 HDD?Or will there be problems?
Now i have a new 250Gb WD sent to me as a replacement can I just plug it in while my seagate is hooked up and use 2 HDD?Or will there be problems?