AMD cpus are sensitive with Memories. Performance do change if you upgrade the memories on AMD based systems such as E521.
Dimension E521 can handle following memories:
-PC2 4300 (DDR2 533, 266mhz, 4.2GB/s, 4-4-4)
-PC2 5400 (DDR2 667, 333mhz, 5.3GB/s, 4-4-4 or 5-5-5)
-PC2 6400 (DDR2 800, 400mhz, 6.4GB/s, 4-4-4 or 5-5-5 or 6-6-6)
-PC2 8500 (DDR2 1066, 533mhz, 8.5GB/s, 6-6-6)
I have tested by using following Memory Brand and modules:
3Gb of Kingston PC2 4300 (DDR2 533)V1.8
3Gb of Kingston PC2 5400 (DDR2 667)V1.8
2Gb and 3Gb of Geil Black Dragon PC2 6400(DDR2 800)V1.8-V2.0
1Gb of Geil ULTRA PC2 6400(DDR 800) V1.8-V2.3
2Gb of Geil Black Dragon PC2 8500 (DDR2 1066)V2.2-V2.4
All memories ran at V1.8 and the PC2 8500 was not recognized by the motherboard. The system BIOS thinks it is PC2 6400. This is not because of the motherboard entirely. The maximum CPU you can put is AMD 6400+ and the cpu can only handle up to PC2 6400.
PC2 8500 memory ran at the speed of PC2 6400 and the memory bench mark didn't show much difference in speed. It was rather better with PC2 6400.
Cas Latency of all memories here are 5-5-5-15 due to the Voltage handicap. But these particular Geil memories also run at the voltage of V1.8 so that's why I bought them. (But having said that, I do not remember the Latency of PC2 4300 memories)
For PC2 8500, the working voltage is supposed to be V2.2 according to their webpage but was still working just fine under V1.8.
All memories should be
-Working Voltage must be from V1.8
Otherwise, I can't guarantee it would work with this system.
You may mix Rams providing they are the same brand and more or less the same speed OR other brand running at the exact same speed.
Generally, there is no harm mixing rams running more or less the same speed but if you mix Ram that has enormous difference in speed with others, you will encounter the performance sluggishness (i.e PC2 4300 and PC2 6400).
My original E521 came with a pair of Kingston PC2 5300(DDR2 667). It was only a GB in total so I added 3 GB of PC2 4300(DD2 533) thus mixed.
The example of sluggishness you will see is the odd system start up time. Sometimes, it starts up faster but sometimes not. You might experience other problems. I have used this combination for nearly 3 month and the mixture didn't actually damaged my motherboard.
I have came up with the conclusion that it is best to UPGRADE to
PC2 6400 (DDR2 800) Memories.
2Gb is quite enough to run Vista and XPs smoothly. It is enough to run Windows 7 as well. 4Gb is fine too.
There was some mysterious event here though. I have been emailed that Geil memory didn't work or any other brand crashes the system randomly.
From this fact, although i have tested myself with the above listed Memories, I can not guarantee which memories work with your system. Some people bought the official memory from Dell and it still didn't work as well.
Apart from the possibility of ''faulty item'', there are other possibilities that choke Dell E521 from working properly as soon as you install the new RAMs.
Memory can be expensive if they are not working properly.
Upgrading the memory might be the most time consuming activity if you are unlucky.
Try the ones i have listed or if they are too expensive, then try the cheaper ''Value'' ones to test out.
Novatech and CCL has a good service on returning and replacing the items you bought. Please do not try to buy the memories from Ebays.