Well sticking with the same brand is ideal but you can mix as long as they are same voltage and timings. 4GB is more then enough ram for gaming 8GB is a big waste IMO for gaming you will never need that much for gaming.
Windows XP was not the best at managing memory and it being 32 bit can only use up to 4GB. Windows Vista and Windows 7 uses up more memory then xp to run thats why you see all new computers with at least 4 GBs. When you get windows 7 make sure you get 64bit to utilize all your memory.