Gecube HD 2600XT

Hi all i need some help on a little situation.

I need to find out whether the Gecube HD 2600XT 256MB GDDR3 will work with a motherboard that has a PCI-E x8 bus speed. The motherboard does have PCI-E x16 slot length so i know the graphics card will fit but i don't know for sure whether it will work in x8 mode or not.

The reason i am unsure about this is because the official website requirements page says this:

"PCI Express x16 based PC is required with minimum one x16 lane graphics slot available on the motherboard."
http://www.gecube.com/products-det [...] quirements

Is it just talking about needing x16 lane slot or does the slot have to run in x16 mode too, i'm just a little confused, i think it will work in x8 mode but need some confirmation.

Also it says it recommends a 400w psu or greater.

The PSU is a generic with 400w and 20A on the +12V rail

To summarize my questions:
Will the HD 2600XT work with this PSU and will it work with this PCI-E x8 bus?
Would the HD 2600XT be bottlenecked by the x8 bus speed to create a noticeable difference in performance when compared to running in PCI-E x16 mode?

Thanks

btw if it turns out that i can't run this card is there any other decent card within a £60 budget