Ouch, that is old. You have an old Socket 478 Pentium 4. This means your graphics options are restricted to PCI and AGP. PCI will bottleneck just about any modern GPU severely, so that's out. AGP has a lot more bandwidth, but AGP cards have been out of production for a while, they are getting increasingly hard to find. The best AGP card that is easily obtainable is the Radeon HD 4670. I think there are still a couple on newegg, but they are quite pricy for their performance, selling for close to $100. If you insist on keeping that old system, that is the best graphics upgrade for you. An AGP Radeon HD 4350 might also do the trick if you want to save some money, I'm not sure how GPU intensive SolidWorks is.
As said above, you're probably better off with an entirely new system. The Pentium 4 is a lot slower than even the cheapest CPUs out there right now. If any of your applications are CPU intensive, you will see a huge performance boost with any new system. At this point the P4s are really only fit for basic web browsing and office tasks, anything more intense really benefits from a newer CPU.