Can we use a PCI Express GPU with a PCI main board ?

In a nutshell, no.
PCIx GPU's will only fit in a PCIx slot.

Same thing goes for AGP, PCI, ISA, etc..

Do not try to force the card into the slot, if it does not go, pull back and check to see why it is binding. Usually the main reason why is the metal bracket attached to the card is not aligned up with the slot it's supposed to into.

The card should go in with a firm motion, and seat solid.

Nope.... pci slots are smaller then pci-e...... sure you might some way get a pci board to fit in pci-e.... but it will never work.....
Your stuck with pci, if you are stuck with a pci motherboard.....
About the best graphics card for pci, would be the visiontek x1300.

pci-xpress x1 slots are currently useless because lack of cards for these slots however pci express x16 slots are currently the top of the line as it is the new technology (a lot better than agp) and there are many cards for theses slots. for the cost no object person who has money i reccomend and nvidia quadro video card, and for the more budget concious (broke) like myself i reccomend a nvidia 7300 gs or better. there is one thing to consider, for example i had a pc i built (traded it off) that was an intel d945gtplr motherboard and a pentium d805 w. 1 gig memory and the gma p50 chipset on that board for video actually far exceeded the recommended video requirements for world of warcraft, so if you are not doing cad work or a massive gamer then the onboard in most pci-e x-16 boards will be fine. if you plan to eventually run sli i recommend a motherboard with nvidia graphics on the board and if you plan to run crossfire find a board that has ati on board graphics. so to sum up,

pci requires pci card
agp requires agp card
pci-e x-16 requires pci-e x-16 card
there are a couple of video cards for pci-e x1 slots but why bother your onboard is probably better than these anyway. if you are looking for a killer board i recommend the intel d945gtplr it has a 775 tsocket for P4 Pd and the new core dou's and the onboard chipset runs WOW DOW and solidworks 06 with no problems. to upgrade the graphics with a card would require at least a geforce 7300gs or better card to actually notice any difference.