you're fine. a pci 3.0 card will work in a pci 2.0 slot.
the chart i posted shows that there is such small difference it is highly unlikly it will affect you gaming at all. and a pci 3.0 card is backwards compatible with a pci 2.0 slot on the motherboard.
since you have a H61 motherboard you do not need to worry about cooling the cpu because that will not overclock and the i3 do not overclock at all.
a crossfire bridge is a ribbon cable used to connect two similar AMD graphics card for a multi card configuration. if you care to read a little more about it:AMD CrossFireX
the card you are looking at comes with one, i doubt you will be needing it.
as far as a PSU; the 7850 is insanely low in power consumption around ~100 watts.
and your i3 2120 with your hard drive, RAM and chipset won't go past another 100 watts for 200 watts total.
(btw, if you didn't already get the card, i'd suggest the 2GB version instead. if you do have it, no worries)
so a quality
350-400 watt PSU is fine but to give yourself a little room to upgrade with:CORSAIR Builder Series CX430 V2
$24.99 after $20.00 rebate(s)
anything past that is completely unnecessary! getting too big of a PSU is as bad as getting too small of one.