Don't change the card mate u got an awesome card and it will work with your board with no problems,HD 4870 is a PCI-E 2.0 card but PCI-E 2.0 is backward compatible with PCI-E 1.0 (Your motherboard) also u will have GDDR5 with no problems.
I think u made a little mistake here,GDDR is for VGAs and DDR is for RAMs so as i said u can take use GDDR5 with no problems
Well, OP, the card will make full use of the gig of ram on the card. The time whin the pcie bus matters on the motherboard is when the cpu is passing of data to the card to set up the scene to be drawn, or if the card runs out onboard memory to store stuff, then it has to put some of that in system memory and go over the pcie bus. Most cards that are pcie 1.0 only had 512 meg on them anyhow and so may use the bus more on newer games with larger textures and more polygons to draw.
In a case like this, more memory on the card will mean it uses the much slower computer ram a lot less, and that is a good thing ! That and when you run a new card on an old system, you can still turn video options way up and not take as much of a hit as if you were using a 7900 gt for example. Your MOBO will use the card to it's (the mobo's)max that it can, even if your not pushing the card as hard as a new mobo and cpu can. just enjoy your card man!