do not get the Intel Extreme graphics card, they are very crap.
Dunno what it is with Intel, they make good CPU's but bad video cards.
ATI Radeon 8500-9700 will all give you good performance with all the RC Flight Sims out there. RC Sims don't have cutting edge graphics so you should be fine with only middle of the range stuff.
For a NVidia Card, skip the crappy MX version of the GF4 as they aren't actually GF4 GPU (rather a slightly beefed up GF2 as they don't have pixel shaders etc). For the money a GF4 Ti4200 is a good buy, and you could run at higher resolutions say 1280x1024 in 32bit.
I run FMS etc at 1600x1200x32 with 2xAA on my P4 @ 2.5Ghz using a GF4 Ti4400 and they look sweet
Get what you can afford, but don't get the Intel graphics.
Also remember if you get a 1.6Gig P4 or greater that you could overclock it later on. Mine is a 1.6Gig @ 2.5Gig, always good to know you have extra speed available on tap.
Buy well

SimJen