Be careful, if you do go for an internal HDD make sure the motherboard supports the size before buying.
By standard most motherboards 1-2yrs old will take upto 200GB. Newer will goto 400GB+
If 3yrs+ Check the specs for your motherboard, if your not sure. There is many ways to find out what motherboard you have.
1. Get Sisoft Sandra, that will let you know the manufacturer and board version etc.
2. Take the side casing off your PC (not a good idea if under warranty - if thats the case then get an external HDD)
3. Get someone else to find out if 1 or 2 fails