I'd say get a P45 board (preferably PCI-e 2.0), throw in 2x2GB DDR2 (4-4-4-12 if possible) and a decent dual-core. If DDR3 is a must, you'd best go with i7, but that would likely ramp up the price, meaning you need to cut corners on other stuff, if it's doable on that budget at all.
GigaByte is always a good choice for boards, but they're expensive. Kingston offers good modules for a good price with maximum compatibility and a lifetime waranty, no strings attached.
Over here, a E8400 costs about the same as an Q8400 (!) (just under 150,- euros). For 10 to 15 more, you have a Q9400 (just under 165,- euros).
Get a case to your liking, make sure it has a decent (preferably
good) PSU.
There, simple.
But...
You mentioned Black Shark. At the moment, single core speed is very important for it, since it doesn't support multi-cores. You can enable it through the OS (set affinity for the process to all cores), but it's less than ideal. With upcoming modules, there should be native support for multi-core and multi-threading. Since your primary focus is flight-sims, I'd say a quad is mandatory, especially with future releases. Since not all sims can use multi-cores effectively, especially older ones, you'll want a fast one at that for high speed per core.
(flight) sims are heavy on the CPU. You want a good one.
