Question : Problem: Best PC6400 memory for Dimension 9200

Here are my computer specs:
Dell Dimension 9200
OS: Windows Vists 64-bit

My question has three parts.

First I had 4GB of PC5300 memory 4 sticks of 1GB RAM.  One pair had timings of 4-4-4-12 and the other pair had timings of 5-5-5-12.  I bought 4 new sticks of 1GB RAM each of Crucial Ballistix PC6400 Ram with memory timings of 4-4-4-12.  My first question is will I get much of a performance gain from going from PC5300 to PC6400?  Let's say I spent $90 on this upgrade is it worth it?  I do mostly gaming and surfing on the Internet and running applications such as office.  Also I notiched that when I upgraded to these Ballistix memory sticks my game started crashing.

Second question is what is the best RAM that I can get for my Dimension 9200?  I know it supports maximum of PC6400 RAM but it still backward compatible with PC5300.  I basically have 5.9's for Windows Experience index in Windows Vists on everything except my memory.  When I had the PC5300 RAM I had 5.6 for memory when I upgraded to PC6400 it only went up to 5.7.  I went memory that is going to take me up to the 5.9 mark and the Ballixtix stics I got didn't cut it.  Please let me know which ones are the best memory I can get to achieve this considering cost as well.

Third can someone explain what these memory timings mean?  4-4-4-12 and so on is lower better still?  What do each of those numbers mean?

Answer : Problem: Best PC6400 memory for Dimension 9200

"My first question is will I get much of a performance gain from going from PC5300 to PC6400" This depends on your processor specs. Your probably not going to find any that will get you a 5.9 as you are limited by your Dell motherboard. I have 4Gb(2x2Gb) of Crucial Ballistix Tracer DDR2 800(PC6400) and get a 5.9. The dell motherboard only supports a memory voltage of 1.8 and the Ballistix memory requires 2.2 to run at max performance. There is a good chance that the memory is also only running at 5-5-5-18 due to the voltage issue and on a Dell system there is no way to adjust as these options are not available. Also running 4 sticks of ram on an unbuffered bus will also hinder performance, you may want to go with 2x2Gb sticks rather than 4x1Gb sticks. Here's a link with more information on clock latency's(memory timings).
http://en.wikipedia.org/wiki/CAS_latency