some questions about my HDD and a future buy

hey guys i was just using my 250gb HDD from seagate and my OS currropted on my 160gb HDD

now i sold my 160gb and installed the windows on my 250gb with leaving all the files as they were

5 days back i got a ram kingston and it did not work on my OS and gave me all types of BSOD so i had to reinstall windows again and my corsair ram worked

note that i did not do a fresh install and just installed the OS leaving all the files as it is so it worked for 2 days but then

2 days back my 250HDD just did not load the windows in safe or normal mode and i was clueless wat happened

so i just formatted my HDD (full format) and have installed a fresh windows for a day now and everything works fast and great

now u guys think my HDD will be fine or i shld just change the HDD?

also iam thinking on adding 1 more 500gb or 1TB HDD to my rig so which shld i get ?

i have 500gb (external maxtor) and 750gb (external) already

any advice?