Hrm... yup.... exactly what I expected when I saw a thread asking for graphics card help. Transitioned instantly to "your gear is horrendous and should be destroyed for the good of human kind"
/eyeroll
Ok, heres my honest help, Scale back what you want to put into the thing video card wise. being a 500 series, that is actually fairly new. You might want to try to bump up to say a 540 or 550, but beyond that it will scrap your budget.
Then take what you saved, and put it to putting your box right at 4gb of ram total, with faster ram. You can get much faster DDR2 ram.
I have a similar box I use for my HTPC, 520 gfx, 4gb ram, Pentium D 3.0 and I can use it to run Arkham asylum, The old republic, New Vegas, and more modern games just fine.
So card wise, perhaps something like
http://www.newegg.com/Product/Product.aspx?Item=N82E16814150615
and
http://www.newegg.com/Product/Product.aspx?Item=N82E16820231144
Would be a MUCH better improvement than a 150$ video card can offer.
Edit: Best offer of help? Dont ask PC hardware questions on this forum. Seriously, it always goes the way of ePeen waving contest built to make you (inadvertently perhaps?) feel guilty about your gear (master race insecurity perhaps?)
In short you will get bombarded with a lot of bad Unrelated information before you get anything even resembling help.
Edit 2: Yeah, Pentium D is the first series of dual core processors. They range from DCs from 1.5-3.0 ghz per core. /eyeroll again.