I am sorry, it is Optiplex 580, did not see many requests for this model yet, but....
First and subsequently second - Optiplex 580 uses BTX standard and therefore is not compatible with modern ATX standard, meaning that you can not move motherboard to the new case, you have to stay with existing one.
Now regarding power supply, lucky you, you have Mini Tower - standard ATX power supply will fit with minor modifications, you would have to bent a few metal parts to advance power supply since it is longer than original, but it fits.
Now regarding video card. Let me explain something.
THIS POST APPLICABLE TO MT (MINI TOWER) FORM FACTOR ONLY.
Unfortunately no card that is more than 7.5 inches long or 2 slots wide will PHYSICALLY FIT inside the case - it was found that only single bracket video card can fit due to BTX MOBO, where card internals facing upward towards PC internals, not ATX, where dual card design is blocking another PCI slot by facing downward.
If you prefer the card with beefier fan, then the length of the card must not exceed 7.5 inches, otherwise only "skinny" single bracket, single slot cards can physically fit.
Pictures below will prove my point.
View shows no room for dual bracket
Single bracket only, or you have to cut dual bracket in half
Here you can clearly see only single place for the bracket
Visual explanation
7.5 inches explained, also shown "obstacles"
This is the picture of actual "skinny" single slot video card (not sure which model)- perfect, "glove" fit.
Now we need to find 40 GBP card.
Now the question still stands, you were thinking about upgrading power supply, power supply costs money, which you can spend on better video card, and this video card would not require power supply upgrade, so how about that?