To be honest any hard drive you get should be very easy to install, just a case of opening your case and plugging 1 or 2 things in and thats it! just need to look at your original hard drive or the manual to see what type yours is i.e SATA or IDE, there just different connections.

And as for how much gb, how many rainy days are there in a year?

Seriously though, depends what your doing on your pc, if its mega downloading you do then id say at least 200GB+, if its only browsing the net and word documents etc then id say 60-80 gb