What power supply are you using? The last time I had a hard drive shutting itself off inappropriately, the problem turned out to be my Apevia 500W power supply, the problem went away once I replaced it with a 450W Antec power supply (this was before I was aware of their capacitors). Maybe your power supply is cheaply made, has insufficient +5V/+12V rail ratings, or is failing.

Exactly what model is your nVidia card? chances are the nVidia card you've been using is a lower-end model compared to your ATI Radeon X1650 Pro.

Prime suspects are PSU and motherboard caps.

The resetting hard drive could be bad power, but it could also be a dying hard drive. However, dying hard drive and bad power can be closely related (the latter can cause the former).
The slow video issue sounds like a driver issue or maybe a card fault. Does the control panel imply that the drivers are working properly, or does it just refer to your card as a generic VGA device?

I don't know much about ATI, but the slow graphics might be an intentional underclock due to the driver thinking that the card doesn't have enough power. nVidia drivers will do this on some cards if the aux power isn't plugged in.

If you have a multimeter I'd try probing the 5V and 12V (I think 12V is probably the only one that's really used) on that aux connector (but if you aren't used to doing this then be careful). Maybe the connector is bad or voltage is low enough to trigger this behavior.
You could also just measure at an unused molex connector instead, which is a lot easier. Preferably one that's on the same set of cables. Even though it's not the point that's plugged into the card, it should give you basically the same readings. Ideally you'd want to watch the voltage(s) under some load, perhaps as the system is booting up.

For a single common cause of both symptoms it sounds like a bad power supply.

shamino wrote:

The resetting hard drive could be bad power, but it could also be a dying hard drive. However, dying hard drive and bad power c […]
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The resetting hard drive could be bad power, but it could also be a dying hard drive. However, dying hard drive and bad power can be closely related (the latter can cause the former).
The slow video issue sounds like a driver issue or maybe a card fault. Does the control panel imply that the drivers are working properly, or does it just refer to your card as a generic VGA device?

I don't know much about ATI, but the slow graphics might be an intentional underclock due to the driver thinking that the card doesn't have enough power. nVidia drivers will do this on some cards if the aux power isn't plugged in.

If you have a multimeter I'd try probing the 5V and 12V (I think 12V is probably the only one that's really used) on that aux connector (but if you aren't used to doing this then be careful). Maybe the connector is bad or voltage is low enough to trigger this behavior.
You could also just measure at an unused molex connector instead, which is a lot easier. Preferably one that's on the same set of cables. Even though it's not the point that's plugged into the card, it should give you basically the same readings. Ideally you'd want to watch the voltage(s) under some load, perhaps as the system is booting up.

For a single common cause of both symptoms it sounds like a bad power supply.

Good suggestions.

Any card in a slot has 3.3, 5 & 12 volts available to it.
(Also -12v but I don't remember if all slots have it. And for ISA -5v.)
Which voltages are actually utilized by a card is up to the card's designer.
It's best to check all the rails including each 12v rail individually if that's applicable to the PSU.

Very rare but I've seen a few cases where bad caps on a mobo corrupted HDD data.
The bad caps were in Vtt (or similar) to the HDD controller and thus data signals passing through it were being scrambled.
(Vtt has to do with the signaling voltage. Each IC can have it's own or they can be shared. Design dependent.)

I've also seen the HDD cycling thing caused by bad power (also rare) but there are other possible causes.
Same with the video. Other possible causes.

With both the HDD cycling and video issues at the same time, bad power is most likely.
The thing to do is find out what is making which power bad.

Waiting for the OPs' visual inspection results.
Checking the DC voltages would certainly help.

Should note however that good DC voltages are not conclusive as to the integrity of the power.
The DC can be well in spec and the ripple way out of spec at the same time.
.

Most definitely sounds like a PSU issue.

Change to a different unit or if you've time, change the caps!