In terms of upgrades and not knowing the system specs.
If he is using onboard graphics , that would be a fisrt to a 512 meg 7900gt are not to bad, but generally CPU speed and RAM will have the greatest affect. If using XP then 2 Gig DDR2 800 MHZ ram or faster will help a lot, vista go for 4, but make sure you get them in pairs so they will operate in dual channel mode, and dont mix speeds it just dont work. CPU a 3 gig or faster will help. If you already have a PCIX video card look at ram and cpu first.
I have personally found 2gb on Vista Home Premium to be sufficient. 3 is my sort of reccomended limit... going up to 4 means buying a 64-bit operating system.
Not all motherboards support dual channel either, so that could go to waste
Remember that clock speed does not equal performance. It is the actual processor itself, not the clock speed that makes them good. A few year old 3Ghz will easily be beaten in performance by a modern dual core 1.2Ghz or maybe even a single core at a bit lower in speed. Though a modern 3Ghz or dual core of around 2Ghz or less should be sufficient for most tasks.
But yes, specs will be needed and you have already covered that one.
Will 2gb of DD2 800Mhz cause an improvement with the graphics? Umm... it depends. It will allow you to have more objects. More RAM means more storage for things. You will be able to see more trees, buildings etc. THOUGH.... the CPU or the graphics card may not be powerful enough to handle this. It is rarely one upgrade that will fix everything. You may need to improve other aspects to get an all-around improvement.
FSX is mostly graphics intensive. I personally have 2gb RAM on Vista Home Premium with a 1.8Ghz Intel 6320 dual core and an 8600GTS. I get medium graphics in typical flight and can get maybe 5-10FPS on high spec. If the motherboard is capable, I would try and opt for an 8800GT graphics card... they are going to give you some very good quality for the price.
So please find out what's in it then we can tell you exactly what's needed.