Some people disagree with me but, it is NEVER a good idea to mix memory types or brands. I certainly would not try to steer you away from buying our memory. However, when mixing memory on current MOBOs, ideally you need to add the EXACT same memory( same brand, same size DIMMs, and same ICs) or there is the possibility of having system instability or a system that will not even boot. Also, in this specific case, there is a good possibility you will be mixing different ICs which is never a good idea. Most MOBO manufacturers recommend against this sort of mixing also. MOBO bios plays a big part in this also.
I have mixed memory and in many cases it will work. However, if any person tells you it will in fact work, keep in mind 2 things.
1. They have most likely never mixed the exact combo of MOBO+bios version, CPU, and memory you intend to mix so they cannot know for sure it will work.
2. The person(s) that give you this advice cannot refund your money or give you new memory in the event that it does not work.
I'm not singling anyone out here, but this post is a good example. I see literally 1000s of posts each week in 20+ forums. Mixing memory is not a good idea. Buy a 2gb kit IMO.