As said above, switch to a triple channel memory setup.. A 3 x 2GB kit will be good.. For the video card, get the GTX 460 1GB if you can.. FSX however is CPU intensive and as such, you'll need as much CPU horsepower as you can get (which further means that you can relax on the GPU by not needing an ultra high end video card).. The i7 950 is good and with a healthy overclock, should be able to handle it pretty well..
thats a fairly old card. I would also suggest a 460 1gb
if you want to go cheap they have deals on the 460 768meg version as low as 130$
I got the hawk edition msi 460 1gb and it overclocks to near 920mhz without really changing voltage etc too much. (209 after rebate)
I think you can still keep it under budget and it will look better.
also the 9800 chip was pretty kickass.. the 4870 isnt much of an upgrade.
still I guess you can start with that and upgrade later.
as mentioned socket 1366= triple channel memory so go 3x2gb chips. preferably try to find a triple channel kit on the approved list for your motherboard and get that. Make sure the voltage requirements are also met (some require too high voltage)
FSX is more dependent on CPU power than video card performance.
People tend to forget that FSX isnt fast paced and anything over 25 FPS is very good - there is no fancy top gun stlye flying - in fact the whole game is based on smooth accurate controls.
any i7 series CPU will be more than adequate (i used to run it on an E8400 with an 800GT) - again like others said 58 chipsets are triple channel so you need a 3x1gb ram setup as a bare minimum, but 3x2gb is prefered.