i5-2400 will be better. However as previously mentioned, FSX isn't all that demanding so the i3 would work just fine.
i5-2400, H61 mobo, GTX 560Ti would be a really nice set up
Jump up the price range a little more, and get i5-2500k (over-clockable), Z68 motherboard and GTX 560Ti.
Tone it down a little, but allow for future upgrades....
i3-2100, Z68 motherboard, GTX 560Ti.
i3-2100, H61 motherboard, GTX 560Ti.
EDIT: If you post us a budget and let us know what websites/country your buying the components from, I'm sure we can come up with a build to suit your needs
a build advice template can be found here: http://www.tomshardware.co.uk/forum/261222-13-build-advice