To capture HD gameplay you may need to install a specialist capture card, which can be quite expensive. What input do you intend to use to capture the gameplay? HDMI, DVI?
For a decent HDMI capture card, you're looking at something like this: http://www.sirentechnology.co.uk/product_info.php?currency=GBP&products_id=550
I can re-work your specification to include this card in the price if you like?