First of all, welcome to the forum, and yes I'm sure we do care... don't we? *angel expression*
Right... Lets see now.
I'd assume, because that the 512MB of ram is running at a faster speed (400MHZ), this would be much better than the ram running at only 133MHZ.
This would be because having both in together, which is what your doing, the faster ram is being slowed down by the slower ram and so it is a bit of a waste...
So yeah, remove the old ram, and just keep the 512MB of ram. Its still a pretty big boost, and you could always go for more after with the same speed. At least you know you wouldn't have wasted our money on the new ram hehe
since even though you'd have more memory, you'd have less of a speed boost as a result.