Well, I like most of the members here tend to build our own. Or rather "assemble" since it has become that easy to do.
The AGP graphics interface is for all intents and purposes dead, as illustrated by the lack of drivers available for your card. DBZ has found a couple of adequate cards, should you decide to go that route.
Your installed memory is really bare bones minimum, for this day and age.
Any modern day motherboard will have superlative sound, without the need for an add in card..
Maybe you might peruse this discussion;
https://www.techspot.com/vb/topic133075.html Before you decide one way or another.
The fact does remain, you can build a pretty slammin' up to date machine in the $400.00 price range. and have something you can upgrade without putting money into an old machine.
This supposes that you are happy with the keyboard, mouse, and monitor you already have. But, doesn't include the cost of a copy of Windows, which would set you back $110.00 or $140.00 bucks, depending on whether you choose home or pro version.
As to the older flight sims, M$ "Flight Simulator 2004" will even run well with integrated graphics, but the newer "Flight Simulator X" is quite a load.
So, figure the costs for what you'd like to upgrade, then decide what you'd like to do. I think you need a video card, a PSU, extra RAM, and perhaps a sound card. Even on sale, but assuming you buy new, this is about $200.00.
I don't have your bills, so it's easy for me to say replace it, but if what is holding you back is uncertainty rather than funds, you'll get plenty of help here to work through that. Maybe even too much.