I don't doubt it. Unfortunately, the 48xx series cards are still very expensive over here, which is why I decided to get this one.
Now just an update on my current problem:
I got a new PSU, a 650W one - Chieftec CFT-650-14CS, the +12V rail amperages are +12V1: 18A; +12V2: 18A; +12V3: 18A. Unfortunately, even if the PSU had anything to do with it, the problem is still there.
And it was the most powerful PSU the store (the same one where I bought the card) had. I've checked other stores before buying this PSU, but they didn't have what I needed and the prices on what they had also differed.
I've also checked ATI's official forums and found some people who have the same problem as I do, but no explanation for it. One guy said that in the end he reinstalled the OS and after he did that and installed the drivers and utilities on a freshly installed Windows XP, the problem did not appear anymore and he still doesn't know what the cause was.
I'm running out of ideas. I use Mass Effect to test whether the problem's still there, because the reboots happen the most when playing that game (they only happen in-game, otherwise everything works fine) and so far the longest I've been able to play was for about 40 minutes before the system suddenly rebooted, this with the latest card drivers and utilities installed. And the GPU temperature after the reboot was usually around 42 ºC (106 ºF) and dropping (though sometimes it was up to 47ºC), but I doubt this temperature would qualify for overheating. I thought about updating the GPU bios, but Asus doesn't have any bios downloads for my card and neither does ATI.
If I don't find another solution, I'll probably have to try reinstalling the OS and everything else along with it. Any ideas?