I'm going to buy a alienware computer that is on sale on the website. it seems like a pretty good deal and with free shipping it comes in cheaper than what the local computer guy can build me at the same specs. the problem is since this system is on sale you can't change any of the compenents and the HDD is only 250 gb. how hard would it be to install a second hard drive in this system considering everything is probably proprietary parts from dell/alienware? would the case have an expansion bay i can put a new hdd into? also how hard of an install is it can i do it myself?

A Model number/name would help...

Most (current) Alienware machines have have a second drive bay, if you have a look on their web site at the machine spec it'll say if there is a second bay available for your model.

it is the area 51 7500 gamers choice

installing drive is so easy it takes 10min or less
step 1ower down PC , unplug all stuff
2: open case
3: get your hdd and sata cable w/e cable it needs
4:set jumper in hdd if you have a main hdd set your 2nd hdd to slave
5: mount the hdd in the hdd bay
6:connect sata cable and power supply
boot it windows will recognize it