which gtx card sufficient for fsx

how can I test if a gtx card is sufficient for FSX accelerator.a grafic card has sometimes 1.0 or 1.2 or 2.0 mb memory.I have gtx460 2mb but would like to upgrade to a faster card.If i do so and the new card is faster but has less mb's than I dont know what will happenwith the fps.I have matrox triplle for 3 screens for fsx in full screen.processor is i7-920 overclocked to 4.1 ghzand with bufferpools on 0 has good framerates.but in heavy area and heavy weather the frames drop and the card has problems to show up the smooth pictures.so my question is : if I buy a faster one and with less mb will that solve the problemand how and where can I test my present mb of the card during a flight