Unless you're into extreme overclocking and need to squeeze every last nanosecond out of the ram, you'll be fine with the same make/model. My 2 512MB chips are still rocking well together.
Bear in mind though that it is possible for 2 sticks of the same make/model of ram to contain different chips under the covers
As long as they can both handle a particular speed and timings you've set, they don't even have to be the same stuff. I used to run 2 totally different chips. Some PC2100 and some PC 2700. Ran then at 147 MHz in my nforce2 without probs for months till I could afford better memory. Better to run similar stuff since their limitation will be similar, but not absolutely needed....