I have a Intel Core Duo 2 running at 1.80ghz, 1gig ram and 160 gig hard drive. I have a ATI radeon x1650 512mb graphics card. 256 of the graphics card is dedicated and the other 256 is shared. Now i am currently playing Flight Simulator X and would love to be able to play the game with all the graphics turned up to full. I was wondering if more ram or a better graphics card would allow me to do this as currently when i try the screen becomes jerky.

Start by adding another 1GB RAM.

You Cant run Flight Sim X at full detail yet. Not even a 8800 GTX Ultra can with Quad core and 4gb of ddr2 ram, Sadly!!
Read a test when they tried using that pc setup and it just cudent get good enough frame rates above 20-30fps.
So sadly you wont be able to run it at full YET!
Altought you can run it better with more RAM and beter graphics card
If you have pci your going to need something in the 8800 series. 8500 or 8600 just arnt worth it!

Core2Duo E6400 @ 3.4ghz
2gb Elipedia 1066mhz5,5,5,15
EVGA 680i
BFG 8800 GTS-OC 320mb
680w Thermaltake Purepower
2 x 250gb Sata
Vista Home Premium