Windows 7 - update BIOS?

will your computer perform better when BIOS is upgraded?

Only update your BIOS if you specifically know that an upgrade will solve a problem. Read the BIOS version release notes to determine this.
You do stand a risk of trashing your Motherboard and the level of risk depends on a number of factors.

+1, I never update BIOS unless it solves a problem, or adds a new feature that I desire (usually doesn't). They mostly update for support of new CPU's it seems. A Guy

Dell provides a listing of what's in each BIOS update. In the past updates added usb boot and wake on LAN, they were critical during Y2K prep.

Yes and no. If your PC is currently working fine, and you don't plan on upgrading any hardware like your Processor, flashing your BIOS will not improve performance.
If you are having issues and a newer version BIOS has a fix for it, then yes it should improve things.
If you plan on upgrading your processor but your current BIOS doesn't support it, flashing to a BIOS that does will improve performance. The performance gain is from the new Processor though, not the new BIOS.
By itself a newer BIOS generally won't improve performance. In conjunction with new hardware though it may be worth taking the risk of flashing it. If you are having issues you may not have a choice if you want a stable system. Just keep in mind that flashing the BIOS is not the same thing as updating a driver. If you get it wrong you could brick your motherboard. If your motherboard has boot block or dual BIOS its not that big a risk. I've done it a dozen or so times without any problems. If you are going to do it, and have never done it before print the instructions out on paper before you start. I've done it from Windows, from the BIOS itself, and from bootable media. Doing it from the BIOS itself if it has the routine built in seems to me to be the safest method.