i'd say if you dont have the money then dont get it, since it will not matter as much if you have good ram and cpu.
you'll still need just one cable with the two connectors on them, then you'll just have to adjust the little pin thingies to set primary and secondary hdds. it should be in the manual
check out www.techbargains.com they constantly have sales on HDD's, from 120 gb to over 300gb with massive rebates. still not as good as black friday tho...
i think there really isnt a big difference in hdd's because i have an 80gb maxtor and it works fine, and another 160gb WD which has no problems, but i heard seagates are supposed to be a bit better?
yeah so i 'd just get the cheapest one for the most gigs