Hi Jake ;-)
well mate its quite simple, lets first find out what type of ram you have by downloading this
http://www.sisoftware.co.uk/index.php?dir=&loc ation=sware_dl&a=&lang=en
now ddr (double data rate) ram is a newer and faster type of memory from sd (Single data rate ).
SDRAM is the older type of memory, commonly used in computers prior to 2002. The more current type of memory, double data rate (DDR) SDRAM hit the mainstream computer market around 2002 and can be found in most new computers. DDR SDRAM is a straightforward evolution from SDR SDRAM. The big difference between DDR SDRAM and SDR SDRAM is that DDR reads data on both the rising and falling edges of the clock signal, so the DDR module can transfer data twice as fast as SDR SDRAM.Now if your 512mb is sd ram then you must use sd ram for your new upgrade as mixing memory generally does not work.
In plain english if you can use ddr memory then qualiteee but very expensive.If not then still go for it but dont mix two diff ones together.Hope thats ok;-)