No it isnt. The Graphics card slot is either AGP or PCIE. your motherboard is probably supporting DDR2 which is not relevant to the memory on the Graphics card. You could run GDDR3/4 or 5.
Why not post your system specs an dwe can go from there.
The GTX480 is overkill for your monitor. The 5770 is really the highest you need to go for your monitor as mactronix and rolli59 said. To be honest, if you're going to spend $500 to upgrade your graphics card, why not spend $200 on the card, and $300 towards a new monitor? I went from a 19" 1280X1024 monitor to a 28" 1920X1200 monitor less than a year ago, and it was the best single upgrade I ever did.