There aren't all that many graphic settings to lower honestly, but I was able to run it fine on my old desktop with a Geforce FX 5200, and 2 gigs of RAM. This new one has a 6150 LE as well, I haven't tried running SH3 on it (yet) but I don't think it'll be much of a problem. I'd try to upgade your system RAM though, if you plan on playing any mods you'll definitely want the extra boost.
Also, are you using an HP slimline by any chance? those system specs sound very close to mine
Like the exact same as mine.