System Manufacturer/Model Number Toshiba A215 OS Windows 7 Home Premium 32bit Memory 2Gb Ram Graphics Card ATI Radeon X1200 Screen Resolution 12800 x 800
Keyboard US Keyboard Hard Drives 160GB Internet Speed Comcast High Speed Internet (running off router)
They didn't slow down a computer in Vista, and they don't in Windows 7 either. If you have a poorly written gadget, I could see it using more resources than another one, but as long as you don't have 30 gadgets running and updating all the time, you shouldn't be seeing a difference.
No. Gadget issues were completely solved in Windows 7. They use memory but even on 528mb RAM machines I've tested I can still run several without bogging down.
I believe you still have to have User Account Control set at least on the lowest level to use them, though.