4HDD/11/40000000: SATA(0,0) .. diagnose and repair?

My 2 year old iMac (out of warranty) is currently still functioning, albeit at a much reduced rate showing the following 'symptoms' ........

- Lately at times the (upper-left) backside of the machine has been burning hot ...
- By typing, right-clicking or starting up any application I get stuck with that annoying apple spinning wheel - even with no other applications running/stealing CPU in the background ...
- Starting up might now at times take up to half an hour ...
- Most of the time it sounds like the HD is reading/writing (small click´s like every two seconds). On starting up applications i.e. Safari it makes a loud and persistent grumling sound, then repeatingly makes shorter grumbling sounds + at bunch of small clicks noise too (Believe me, this wasn´t allways a noisy machine.)


Tech specs are ........

27" iMac running on OS X 10.6.8
2.93 GHz Intel Core i7
1 TB WDC WD1001FALS-40Y6A0 (200 GB used)
16 GB 1333 MHz DDR3


Here is what I tried out so far ........

- Running the MacKeeper cleanup
- Running the Memory Cleaner: returns +/- 14.6 GB free memory
- Removing all login items
- Enabling the Firewall
- Uninstalling some software known to cause trouble ('Silverlight' to be specific)
- Cleaning up my desktop to zero files
- Disabling all live updating of smart playlists on iTunes
- Uninstalling all widgets
- Running OS X Install DVD Disk Utility Permissions: did several repairs, but there seems to be just as many faults after as before the repairs.
- Running OS X Install DVD Disk Utility Harddisk: '1 error'. I ran a repair, and since then it has returned '0 errors'.
- Doing PRAM og NVRAM reset
- Copying all data to external harddrive using Time Machine, then erasing the HD, doing a fresh OS X 10.6.4 install and upgrading to 10.6.8

Realizing I might be stuck with a HD issue not detectable by Disk Utility, I ran the Apple Hardware Test which returns error 4HDD/11/40000000: SATA(0,0).

Question 1)

At the time being I do not have the money to just drop it off at the computer store, leaving engineers to figure out whats wrong and what needs to be repaired. So since my iMac has not completely died on me yet, I´d like to maybe closer identify the problem and - if in any way possible - do the repair myself.

- How could I go about this?........

And another thing ... Forgive me now if this question is foolish, but I really am a complete newbie to all of this technical stuff, so here goes ...

Question 2)

I have an external harddrive (Intenso 6002541 640GB Memory Station USB External Hard Drive) currently left with 450 GB free space as my Time Machine Backup takes up about 200 GB space. - Would it be possible to just use this external harddisk as primary HD - running all applications from there - untill further on?

Thankyou for reading. I hope someone can help ...

/malou