Whoops. For some reason I read the Windows as XP. My bad.
As for the GPU, specs don't tell the whole story. The archetecture is a lot different, which allows for better performance on "equal" specs, considering all factors. While they might be equivalent in terms of raw power, the GT 430 has a boat load of other benefits that the 9800 can't claim. The 430 consumes a lot less power, produces a lot less heat and runs quieter. Factor in the added benefits of 3D support and DX 11, and the GT 430 is miles ahead. "Performance" doesn't just mean straight up FPS, it also needs to consider other factors, such as prices (irrelevant here), power consumption, heat production, noise produced, and useful features. I will say I've always been guilty of lumping all those factors in without explaining that.
Besides, the GT 430 is not even the right card for a low budget build, as the HD 5770 would be the ideal choice in a low budget gaming build, but that's also besides the point.
I will also say that trying to sell a complete custom build isn't the best way to get the most for your money. There just isn't that big of a market. The kind of people who would buy a custom build would either want something cutting edge or older, but super cheap. You're build isn't cutting edge, and you're not making it cheap. That makes your competition the lower end Dell, HP, CyberPower, and other prebuilt vendors. Who would you rather buy from, some random guy on the internet who has used the system for a few years, or a "reputable" company that makes 1,000s of builds a year? It's a no brainer.
You'd do a lot better by selling individual parts. For example, you'd probably get $50-60 for the RAM by itself (DDR2 is in higher demand), maybe $100 for the CPU, likely another $40-50 for the GPU (possibly more from someone looking to SLI it), and probably another $75-100 for everything else together (maybe more if you included the case). That'd net you $265-325.
If the OS isnt an OEM version (i.e. you can install it on a different motherboard/computer), add another $50 to that total as you could part it out by itself. Then you'd be getting more like the $400 you want (or the $370ish excluding the tax).