Believe it or not, I run all of the IL-2 series except IL-2: 1946 on my "Legacy" gamer, which is a Pentium III (1GHz Coppermine) with 512Mb RDRAM with am nVidia G-Force 256 video card and Win98SE for an OS. Once in a while, it'll stutter if there's a heavy graphics condition, like being over a large city with flack and a large amount of aircraft, otherwise it's extremely smooth, even when I'm in multiplayer with friends.
I have kept the ol' legacy machine around because I have all of my old games on it (Mechwarrior, Red Baron, Jane's, Air Warrior, Aces of the Pacific, etc etc) so you shouldn't have any difficulties with the first couple releases of IL-2.