How would flightsim handle on this?

System specs:

processer: 1.4 gigahertz
ram: I think 512 to 1 gig buy I not shure...
graphics card: Nvidia Gforce 4800GX or something like that
memory: 20-30 gigs

Would this work well for fightsim?

It'll work fine, you won't be able to run it full out just at default. But...I can't even find that card, if it came with the computer it's very old and will hold you back.
You should think about getting a mid range new card, maybe $150 if you have an AGP socket.