If I was looking to upgrade I think I would bite the bullet and buy the bits myself, it may seem daunting but it need not be.
When you upgrade a video card etc or reinstall windows after a format/new hard drive you just install motherboard drivers etc etc so thats not hard. I have fitted memory, hard drives, video cards etc and replaced motherboard fans, as long as you take your time, take digital images of stage by stage you should be ok and will save a lot of money. The two links you sent were not great machines.
I would go for the nVidia 9600GT card at least, this is a good card and can be got for less than £100 it will eat FS9.1/FSX I would go for a duo core or quad core CPU the bigger the better depending on budget. A couple of SATA hard drives so you can have the O/P on one and FSX/FS9.1 etc on the other.
I think a look at overclockers.co.uk or ebuyer.co.uk will surprise you what you can build for your money...
Oh and building your own means you can whack Windows XP on instead of that fat assed memory hogging bloated Vista

Just my opinion ;-)