The shared video memory is indeed part of the problem (part of what's causing low performance in games)... but it's likely not the main issue. The 7300 is a low-end graphics processor...not so much designed for games (even though they may represent it that way for marketing purposes). Also, the CPU and the speed of the hard drive very likely have a significant impact, along with the additional processor load created by onboard sound, and the total system RAM (as rhagz pointed out).
As was said, there's not much you can do. Some Dell laptops can be upgraded to a higher-end video card, but I'm not sure if yours is one of them. The price will likely be very high for what you end up getting, if it is possible to upgrade.