ATI 4870 512 mb power cable confusion

Hi all,
Have just upgraded from a 8800GTS 320mb to an ATI 4870 512mb.I have changed cards several times before,but have hit a snag this time.My old 8800gts only has one 6 pin power plug but the 4870 has 2.I have tried it in both on separate occasions but just get a power input error message on my monitor.
I have what looks like an adapter lead included but don't know which lead if any they would go in to.I don't want to get it wrong and fry it!
In my PC case is a mass of wires held with cable ties,and there are 3 spare 4 pin plugs if this helps? any help would be massively appreciated indeed!
Motherboard is an ASUS P5N-E SLI n Force(C55) Core 2 Duo.
ATX case +550W PSU
4 GB RAM
Vista 32

What is your PSU ?

550 W PSU sisley

I'll make a bet here...

That power adaptor goes to the same molex (power thingy) that goes from your PSU to you CD/DVD ROM or even to you HDD if it is SATA with both connectors. Also, the Y adaptor has 2 of those molex'es.

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That's the molex i'm talking about (kind off, lol) that goes to the CD/DVD ROM. The adaptor should have 2 molex with that form: female and male. Plug the correct one to the one that goes into the CD/DVD ROM, and the other one to the power cable. The last molex (the 6 Pin or 2x3 Pin) goes to the card like you already know.

Hope it works.

Esop!

It would be really really hard to get them wrong actually... All power connectors (molex'es) from the PSU are different from each other when they have different power throwoutput, even some with the same power, have a different design.

In the worst case, your card won't get enough juice and still complain.

Esop!

Ok so I decided to bite the bullet and plug it in.It fired up first time and seems to be working well on Warhead/clear sky and COD4 for over an hour now.Thanks so much to Yuka and Swifty for all your help.It was really appreciated and I'm grateful for your time.