Upgrade for Dell Dimension 8400 video card

I desperately need help to choose a good video card upgrade for my PC. I have an old 2004 version of Dell Dimension 8400 which came with video card: nVidia 256MB PCI Express x16 GeForce 6800 GTO. But this video card has been replaced at least five times in the past 3 years free of cost by Dell because of the guarantee. The problem will come when the guarantee expires. Since the spares don’t last long, I am seriously thinking of upgrading my video card. COULD ANYBODY SUGGEST A SURE AND GOOD REPLACEMENT FOR nVidia 256MB PCI Express x16 GeForce 6800 GTO – a card that has been tried and found trouble-free to work with the Dimension 8400 system. I enjoy working with Photoshop and Google Earth. My kids, for whom the PC was originally meant, gave up gaming and video editing because of frequent breakdowns. I am not a technical person nor do I understand much of tech jargon and hence am unable tell in depth about my system (especially motherboard). I live in Rome, Italy, where there might not be a wide array of video cards available as in the US or UK. Do suggest more than one so I could at least find one.
Someone suggested that a LOW-PERFORMING POWER SUPPLY UNIT (PSU) is the root cause of frequent burning out. If that's the case could any one suggest a possible PSU upgrade that would fit into the Dimension 8400 tower case without having to move around or replace other components?

Thanks!
Robin
(My email: robin_gomes@yahoo.co.uk)


Below are some specifications that I picked up from Dell support at: http://support.dell.com/support/ed [...] wp1052308.

Model: DIMENSION 8400 P4 PROCESSOR 540 WITH HT
RAM: 4 (1gb x 4) GB PC-4200 DUAL CHANNEL DDR2 533MHZ (detects only 3.2 gb)
Video card: nVidia 256MB PCI Express x16 GeForce 6800 GTO
OS: Windows XP Pro

Components:
1 HDD (160gb x 2)
1 DVD
1 Disk drive

Computer info:
Chipset - Intel 925X Express or Intel 925XE Express
DMA channels - eight
Interrupt levels - 24
BIOS chip (NVRAM) - 4-Mb
NIC - Integrated network interface capable of 10/100/1000 communication.
System clock - 800- or 1066-MHz data rate (depending on your processor)
DC power supply:
Wattage - 350 W
Heat dissipation - 460.7 BTU/hr

PSU: (info gathered from PSU label) Model N350P-00 A00119
Output 350W MAX. +5V – /22.0A, -- /18.0a, – 18.0A, -12V -- /1.0A, +3.3V -- /17.0A, +5VSB –/2.0A,
MAX. COMBINED POWER ON +5V AND +3.3V OUTPUT IS 150W. MAX. COMBINED OUTPUT CURRENT ON + 12VA & +12VB OUTPUT IS 28A. +5V & + 3.3V IS 150V. +12VA & +12VB IS 28A

Like this one: http://www.pcpower.com/products/de [...] index.html