Say you have AMD Athlon X2 4600+ right now. I would definitly upgrade to Athlon X2 6400+.
In this case, upgrading to 5000+ or 5600+ doesn't show much improvements. K8 model windsor and Brisbane family CPUs has it's own limit. Even for Athlon 6400+, it is not fast enough compared to the modern dual core such as Kuma family (socket AM2+, k10) but you can definitely expect a significant improvements if your current CPU model number is below 5400+
Upgrading from 5000+ was also effective. Upgrading from 5800+ doesn't show much difference. BUT,
I strongly recommend everyone to upgrade to AMD athlon 6400+ while the stock last. If you prefer not to spend more than £100 to upgrade E521, then I will recommend you to buy a new system later on. The average cost of building the new rig is about £600 without the monitor today (and this price is for starter gaming machine)
Finally, I suggest you pick up non Black edition of AMD Athlon 6400+. This is due to the fact that you can not overclock your cpu more than you think.
And also, the multiplier of Black edition is locked to 16X so there is no point at all. This is all due to Dell BIOS and it doesn't allow you to overclock.
If you have to pay more on the black edition, then it is better off buying a non black edition. If you are intending to use the black edition for later new rig then it is ok too.
This webpage is created assuming you are going to upgrade your CPU to AMD 6400+ .
But in case you have bought AMD 5800+ because there is not AMD 6400+, I suggest you to upgrade your heatsink to Option 2