Wow. That's a pretty outdated machine. My initial impressions is that your video card is probably the biggest bottleneck, but the CPU is also going to hold you back because SC II is pretty CPU intensive, as are most RTS's.
That 1GB of RAM is probably holding you back as well. If you want to verify this for sure start SCII and then switch back to the desktop while SCII is still running in the background. Open task manager and check to see what the physical RAM usage is.
I honestly would recommend upgrading to a new PC, budget allowing. There are so many parts on that PC that are holding you back. The other option would be to get a cheap video card as an upgrade and hopefully that would give you a boost. However, you may still be bottlenecked by other components and your CPU upgrade options on that PC may be pretty limited.