I've be trying to investigate what effects the performance of FS2002 more; RAM, Video Card or Processor Speed. About two weeks ago I was running FS2002 on a Duron 700MHZ with 384MB of Ram running a nVidia Geforce2 GTS 64MB. With the display settings at moderate with 100% default AI Traffic I was seeing 15-22fps, not bad. However if you are interested in running custom AI Traffic expect to see a slide show - around 4FPS. Just recently I purchased a Barebones system. To be exact I purchase an AMD XP1900+ with the ECS K7S5A Motherboard. Put all my current components into my new case, one problem. My motherboard only accepts 2 sticks of SDRAM. So I stuck with 256 until my 2 512 DDRAM sticks arrive. Well I decided to install FS2002 and see how it look. With Very Dense (40+ aircraft) Custom AI Traffic at KDEN, scenery setting maxed out I was seeing about 10FPS. In conslusion, I have found that Processor speed has a lot to do with the framerates. When my extra DDR-RAM arrives I will be able to give you a better idea.
IMHO I think you should consider purchasing a Barebones system. You can put the components of your current system into the Barebones system. Again, barebones systems include the case, power source, motherboard, processor, heatsink, and cooling fan. You (or a friend) installs the Hard Drive, floppy, CD-ROM and additional cards. If you are on a budget Duron systems are the best. Decent Barebones setups can be had fairly cheap at http://www.pricewatch.com/menus/m235.htm