It really depends on what you do with your computer. Most games don't even need 1Gb, so you'll be fine with just the 1Gb stick. For what the various applications are concerned... well... it really depends on what those apps are and how many you run at once. If you run lots of heavy programs at the same time, you may prefer capacity over speed. If not, speed might be the thing to go for.
As you don't know what your bus speed is (most likely you have a I855M chipset, which does DDR at 333Mhz (PC2700)), it might be a good idea to test some first. If the bus speed is always 266Mhz, than it's a better idea to leave both sticks in. If not, re-read my story above