Hi guys, I'm new around these parts so be gentle 
I've recently noticed a couple of issues with my PC but first some background. The PC is a HP Pavillion Somethingorother and is 3 years old. It has a triple core AMD Phenom 8750, 4GB of RAM, 300w PSU and a Geforce GT 120. Yeah I know, quite the beast.
Recently I have noticed periods of severe fps slowdown when playing games that last for a couple of minutes at a time. These seem to happen regardless of the game (I've experienced it whilst playing WoW, Orcs Must Die, Red Faction, Neverwinter Nights, The Old Republic MMO and the KOTOR single player RPG). As mentioned the low frame rate will last a couple of minutes and then sort itself out. Sometimes minimizing the game will speed things back up quicker.
A friend at work mentioned that it could be my graphics card overheating, then when it drops in temp the framerate resumes. I booted my PC with the case open and noticed that the fan on the graphics card rarely spins, when it does it does so in a "drunken" and very slow fashion. The fan will move a part revolution and then stop, then move again, maybe managing 15-20 RPM.
To add to this yesterday upon booting the PC informed me that my hard disk would fail imminently. I have backed up all of my docs to a second drive however my PC neither came with a copy of Windows, a recovery disc or the standard HP recovery disc creation program.
I have neither the funds to buy a complete system nor the ability to obtain finance however can afford to buy components every month or so to upgrade my PC.
Onto my questions if I may:
1. What is my best course of action to maintain a working PC?
2. Following the stabilising of my PC and assuming that it is up and running as per the spec posted above; in what order should I upgrade components?
many thanks

I've recently noticed a couple of issues with my PC but first some background. The PC is a HP Pavillion Somethingorother and is 3 years old. It has a triple core AMD Phenom 8750, 4GB of RAM, 300w PSU and a Geforce GT 120. Yeah I know, quite the beast.

Recently I have noticed periods of severe fps slowdown when playing games that last for a couple of minutes at a time. These seem to happen regardless of the game (I've experienced it whilst playing WoW, Orcs Must Die, Red Faction, Neverwinter Nights, The Old Republic MMO and the KOTOR single player RPG). As mentioned the low frame rate will last a couple of minutes and then sort itself out. Sometimes minimizing the game will speed things back up quicker.
A friend at work mentioned that it could be my graphics card overheating, then when it drops in temp the framerate resumes. I booted my PC with the case open and noticed that the fan on the graphics card rarely spins, when it does it does so in a "drunken" and very slow fashion. The fan will move a part revolution and then stop, then move again, maybe managing 15-20 RPM.
To add to this yesterday upon booting the PC informed me that my hard disk would fail imminently. I have backed up all of my docs to a second drive however my PC neither came with a copy of Windows, a recovery disc or the standard HP recovery disc creation program.
I have neither the funds to buy a complete system nor the ability to obtain finance however can afford to buy components every month or so to upgrade my PC.
Onto my questions if I may:
1. What is my best course of action to maintain a working PC?
2. Following the stabilising of my PC and assuming that it is up and running as per the spec posted above; in what order should I upgrade components?
many thanks