pci-express 2.0 cards work in v1.0 slots so they'll work in your PC.

keep in mind you'll need to replace the power supply since HP's 460w unit is too small for the cards you mention. a 550W unit is recommended for the GTX 280/285 while a 680W unit is recommended for the GTX 295. GTX cards are 10.5" long so make sure one will fit your case.