Power problems with new x1950 PRO

Hiya I've just upgraded my computer (well essentially got a new one in the end!) and have just bought a new graphics card - an ATI X1950 Pro 512mb PCI-E. However, when I put the card in and power up I get a blank screen with no signal output from the card, and the pc doesnt boot up :cry The card works fine in a different pc so thats eliminated that factor. I've narrowed it down to either the motherboard (unlikely) or the power supply (my money is on this one). After getting a series of emails from ATI, I finally got a useful saying that the minimum specs for the X1950 Pro are '450W power supply or greater, 30 Amps on 12 volt rail is required (assumes fully loaded system)'. My PSU was only 400 Watts so I upgraded it to a 500W one (http://uk.farnell.com/jsp/search/productdetail.jsp?sku=1277267&AFC-GOG-PROD&ATT=1277267)and I'm having the same problem :-( , though it only produces 28 amps at 3V and 25 at 12V.
I'm not entirely sure what the 30Amps on the 12 volt rail is about :confused as this seems to vary between PSUs (I spotted a 1000W one with less than 30 Amps at 12V) but would I be correct in assuming that generally the cheaper PSUs can have a high wattage but be lacking with the current?
I've even tried unplugging the power for everything except my motherboard and graphics card with the same problem (leading me to suspect an incompatibility problem with my motherboard?).
I'm upgrading from an nvidia 7300 LE which is also PCI-E and works with no problems so the port is evidently working.
I've encountered the same problem in a few other forums across the net but no satisfactory answers though they all swayed towards a power problem. Its odd as some people report using the card with less than 30Amps and 450W (the minimum requirements on the box are 350W!). I'm confused!
Any help on this would be greatly appreciated as I'm getting desperate now! I don't really want to get another PSU (with more amps) to find its still not working!

My specs are (to the best of my memory):

Motherboard: ASUS P5VD2-MX SE
CPU: Intel Pentium D 2.8GHz Dual Core
RAM: 1.0Gb DDR2
GFX: ATI Radeon X1950 Pro 512Mb PCI-E
PSU: Colorsit 500W
HDD: 1x 320GB SATA
1x 200GB SATA
Misc: 1x DVD-RW Drive

More detailed specs are available if required!

Cheers,

Oli