Looking good so far, Nathan.
DDR3 RAM is confirmed on the mainboard and CPU, speeds of 1066 or "PC3-8500U" are supported but you can chose any fast DDR3 module which comes in at low prices.
As 1333 or "PC3-10667U" come at the same price as the slower ones, you can chose them to have some overclocking headroom if you want to go for it.
Go for at least 4GB, better 8 and install a current x64 operating system then. A Win7x64 Home or so will do fine. No expensive versions needed.
Your CPU won't kick bu.. in FSX but of course can run it, overclocking is recommended then, otherwise you are at the lower end of being capable to run detailed sceneries together with some complex birds.
Going for overclocking means that you should look for a good and not to pricey cooler then although the Athlon II X2 run very cool and haven't got a huge cooling demand.
As you said that it has to be a cheap HDD, SDDs don't play a role here, they are to pricey then and FSX installations usually grow and grow. So take a current 1-2GB variant with 7200rpm and you should be ok.
SDD performance in case of fps is almost not present in FSX, so you just have loading times to fight which is no problem on current discs.
You don't have to go for special SATAIII or "SATA 6Gb/s" models as they don't use the bandwidth of the very modern interface, so SATAII or "SATA 3Gb/s" is very much ok for normal HDDs.
Of course, if the price is the same, chose the SATAIII one.
A network card is included on your mainboard, even twice.
Quote:2 x Realtek 8111D chip (10/100/1000 Mbit)
No need to buy one. See here for the rest of specs of your board if questions arise
http://www.gigabyte.com/products/product-page.aspx?pid=3258#spAs for the monitor, your preference counts. I think that 24inch models have a nice price point nowadays. The cheaper models use TN-panels which are ok but may be less useful if you are looking for a true color representation.
The magic numbers like 1ms reaction time are mainly marketing ones, you won't find any bad monitors for us "slow" flight sim fans, shooter guys might have another preference then.
You should look for a full HD model, so the resolution will be 1920x1200 and your graphics card won't have problems with this when running FSX.
Other games might change this (performance) picture but a HD5750 can surely run some detail without struggling. Depends on the game of course.
You should choose some price watch site to find your new stuff. Not only watch for prices but also for the reputation of the shop and look at the details before buying.
You can also go to big discounters if the run a shop nearby, all other solutions can be fine too but will raise the price on some parts.
If you need some service though, chose the friendly shops of course.