trust wikipedia has the answer, basically it is a lower power lower heat version of memory based on the DDR2 specification. GDDR3 also features higher bandwidth than traditional DDR2.
so the difference between GDDR3 and DDR3 is that GDDR3 is memory desgined for use in graphics cards and is based on DDR2. DDR3 is the sucessor to DDR2 and is used for computer ram. DDR3 is used with the new intel i7 processors and the AMD Phenom II processors. typically DDR3 has much higher frequencies (around 2000mhz are avaliable now) slightly higher timings of around 7-7-7-20 compared to DDR2's timings of 4-4-4-12. also DDR3 has much higher bandwidth than DDR2 however the increase in actual real world performance is realtively small.
without knowing what your using the computer for its a bit hard to recommend a card. however if it must be a 9 series a 9800gt seems to offer good value for money. but i stress that more information will help us recommend a grahpics card