Well, it really depends on your habit of buying staff.
Like me for example, I tend to buy new mid range computers and I always seeks for upgrades. I enjoy thrill of buying staff before I do something big.
I have a good news and bad news for E521 users (including me).
Good news is that once you have upgraded your E521 fully as I have suggested in this webpage, you don't need to buy another computer for the next , say, 4 years. This is because of the fact that it performs really well.
AMD Quad cores weren't that impressive. I was expecting to see huge difference in performance overall with my E521 but no it didn't. It was only capable to handle multiple applications in stable but nothing else.
Certainly from this point, I can say that AMD 6400+ is such a great CPU.
(Although other Quad core users might shout out about the benchmarking but who cares if we don't see the improvements with our own physical eyes)
Oh I forgot to mention that AMD quad cores produce a lot of heat more than AMD 6400+. ( this is because AMD used the advantage of 45nm process to the clock speed increase while reducing the voltage).
The important thing is that both AMD Quad cores (940,955 and 965) and AMD 6400+ didn't show much difference in performance when I used Windows 7 which is a good news. (I can not tell about XP and Vista right now)
The rest of the performance is up to the graphic card you bought for E521.
Please ignore people at the forums who mention about the bottle necking business of E521 motherboard with modern GPUs. Modern GPUs (such as ATI HD4890) works perfectly well.
The bad news about E521 is that it consumes a lot of energy compared to the modern AMd quad core even you turn on the AMD's Cool and Quiet Technology.