the cheap "dodgy" drives on ebay work just fine and cost half as much, that's what i did, i got 250gb for 60$ it arrived in 3 days from a US shipper with 100% feedback. have not had any problems, nor have my friends who went the same route...
there is also a tutorial online for re-using an old xbox360 drive in a slim for an even cheaper alternative. the only reason i went with a 250gb is the price difference between a 20gb and 250gb was like 20$
my 2 cents, but i can now play all the halo, COD black ops, etc that my heart desires without any issues...