the RAM im using now is SDRAM DDR2 533 (PC2 4200) and I am wondering if I take out that stick and put in two sticks of this... 2GB (2 x 1GB) 240-Pin DDR2 SDRAM DDR2 667 (PC2 5300) Desktop Memory - Retail... will it run?
As long as it's DDR2, there shouldn't be any problems. If you know your motherboard model and manufacturer, you might want to check their website for a list of compatible RAM. Though basically any DDR2 RAM should work, it's always best to get RAM with guarantee compatibility
It will work, but whats the point? Unless your current RAM is fried, stay with your current one. Check out some benchmarks - you will find that the performance difference between PC4200 and PC5300 is quite negligible.