If you have 256mb of 133 ram, I'd say forget about the other 64mb of 100 ram, you won't really need it. If you want to mix and match them, you'd have to step down the 133 ram to 100 in your bios settings so you lose any benefit of the 133 ram. Might want to check on the web and see if there's anywhere that will buy back your 64mb of 100ram (I believe
www.memorybroker.com buys back ram, not 100% sure though).
As to which RAM is better, well... I'm not fully sure there is that much difference in brand names. I've always bought the cheaper "generic" brand and have never had any problems. I'm running 128mb of 'generic' 133 ram right now and have 0 complaints, but then I haven't compared it to name brand ram. I truly don't think it makes much difference in this game.