since you don't have a public beta available, about all you can do is try to find folks with low end gear to test it for you, or put together some sort of stress test program folks can DL and try. all it would have to do would be to test those things you were worried about.
to avoid these issues i always have a low end target platform available for testing. but my projects are for profit, not fun.
for things like memory, you can do dummy allocations to use up ram, making it seem like there's less ram available. this would decrease the point at which windows ran out of physical ram, and had to switch to paging. similar things can be done loading dummy textures and meshes to use up graphics memory. these methods can emulate a machine with less ram.
you can also add pauses to the main game loop to burn CPU cycles. this emulates a slower CPU.
you can draw stuff more than once to emulate a slower graphics card. draw everything 2x = graphics card that's 1/2 as fast.
but it also sounds like you have access to a low end machine (your tester with problems).
since your project has started to go a bit over the top, perhaps you should step back, and decide where you're going with it, and more specifically, what your target platform will be going forward. even if its just the higher end platforms you have available.
if its purely for fun and experience, you may just want to use your PC as the target platform, and not worry about lesser hardware.
issues of compatibility with low end user hardware are only a concern if you're worried about popularity or profitability.
OTOH, learning to deal with low end hardware _would_ be a good lesson to learn if you plan to make more games.
"DirectX is like a belt fed machine gun, where every texture change is like hand loading in a new belt of ammo. worse, every mesh (vb) is a new belt of ammo, and a texture is like breaking the gun down, and setting it up again elsewhere, then loading it, then spraying triangles again. so you want to setup the gun once, string all your belts together, load it once, then just spray."
Rockland Software Productions
"Building PC games since 1988"