My 3-year old GeForce 2 died yesterday, so I had to find a replacement. In the local stores, no one had a GeForce 3 or 4 (except MX series GF 4), so I wound up getting an Nvidia FX 5600 with 256 MB onboard RAM. It turns out to be a good choice. My system is as follows:
Alienware 900 MHz Athlon Thunderbird
256 MB RAM
FX 5600 video card (formerly GF 2 GTX)
After an intense multiplayer session yesterday (after installing the new card, of course), I can say that the 5600 is a definite help, even on my 3-year old system, and that's with FS2002. For testing, I ran with frame lock at the default 25 fps, instead of the 20 fps I'd been using before. Frame rates were very slightly up (1-2), but the overall experience was MUCH smoother. Stuttering in turns is gone. Much less blurring (usually none). Flying an aircaft that formerly brought FS to its knees now is somewhat useable- used to get 6-8 fps, now get 10-14. Better yet, when my multiplayer partner was using the same aircraft, and when he was in view, I used to get 3-5 fps- now it runs 7-11.
After flying this way a while, with identical settings that I used with the GF2 (except frame rate lock, as above), I changed the autogen display parameter from dense to very dense. I then got an occasional brief stutter, but not nearly as bad as I used to have with the GF 2 on the lower (dense) setting.
I also noticed that in areas with dense add-on scenery, the frame-rate hit was less, and operation was more smooth.
So if you need an upgrade for an older system (with AGP), the FX 5600 seems to be a good choice, with an overall improvement in smoothness and scenery sharpness. The more difficult stuff improves more than the simpler stuff (aircraft and scenery). My additional expectation is that, since FS-ACOF (AKA FS2004) apparently uses DirectX 9, that I'll gain additional benefits when that program comes out, since the 5600 is DX 9 compliant.
All the above experience was with the Nvidia drivers that came with the card on CD. I'll eventually update the drivers, but with the improvements I've already seen, I'm in no hurry.
Hope this experience helps someone else.