The GTX 750 is faster than the GTX650, about equal to the GTX650Ti and slower than the GTX650Ti Boost.
A 450 Watt PSU should run any GTX750 out there easily, the card only draws about 60 Watts at full load.
As long as the motherboard has a vacant PCI-E x16 slot the card will fit and run properly.
One of the single fan versions would be the most compact: http://www.overclockers.co.uk/showproduct.php?prodid=GX-187-GW&groupid=701&catid=1914&subcat=1854
A few places to look: Aria, Eclipse Computers, Novatech, CCL, Dabs, Maplins, Ebuyer, Scan, Pixmania and Amazon.
Prices and availability change by the hour!