Generally you don't have to buy the same brand, possibly the only thing to consider is not buying generic (unbranded) RAM.
This is where you have to be careful, your old system is likely to be DDR not DDR2, which is why I suggested visiting crucial as they have a tool to identify what RAM and motherboard bus speed are in your system, from that information suggest compatible RAM for it. Mind you possibly the most important bit is the PC3200 as you could essentially just do a search on that 1gb PC3200 so providing that is in your search string you shouldn't go wrong.
Once armed with that information then you can go shopping for RAM of that specification of any of the major brand names, crucial being one top brand, Kingston and Corsair are more value brands but still up to the task.
These (3.0-3-3-8 @ 200 MHz) (2.5-3-3-7 @ 166 MHz) represent timings if the RAM is used at either 200 or 166 MHz, it isn't really anything to be concerned with if your mother board was only 333 MHz then the RAM would be running at 166 MHz.
See this example search of froogle (google shopping), this is localised for the UK, there may be one for Europe or Slovenia,
http://www.google.co.uk/products?q=1GB+PC3200+DDR+SDRAM, this is an ordinary google search,
http://www.google.com/search?q=1GB+PC3200+DDR+SDRAM.
Used RAM can represent good value, but you have no warranty so it could work out an expensive bargain.
