How much memory can I have?

So I have 3 slots in my motherboard.
2 are already used (2x128MB SDRAM)
What would be I would use all 3 (+256MB)
Someone says what it's wrong, others say that nothing bad will happen. So can someone tell me the truth in facts and numbers?

It depends on your motherboard. What brand and model is it?

Since you say you have SDRAM, you are probably able to use all three slots. Depending on the motherboard, you may be limited to 256MB in each slot for a total of 768MB. If it is a recent enough board, it should take 512MB in each slot for a total of 1536MB.

If you are using Windows 98, then you should not bother with anything over 512MB TOTAL. Windows 2000 or XP will handle up to 2GB of ram.