Although you may not see a big difference in games, you will notice a big difference with an X2 in terms of multitasking and cpu intensive programs. Things like virus scans, encoding to PDF, or transcoding mpeg/etc, or just having ten progtrams open at once tends to bring a single core down to its knees. Not the case with a dual core.
In terms of system preparation it should be a relatively painless process as long as you do it in the following steps:
1) If you have the Athlon64 driver installed, uninstall it --**very important or your system may not boot into Windows after swapping the chips**
2) Go into your BIOS and note your settings, specifically your selections for Power Management (s1,s3 or both), Cool & Quiet, your ACPI setting, and your APIC mode.
3) Flash your BIOS to a version that supports the X2 3800+ then verify the system still boots into Windows OK with the old chip. If it doesn't you will need to go into your BIOS and reset the settings you noted previously.
4) Install the chip, if you plan to OC -- at the very least install a pure copper HSF with some Arctic Silver
5) Boot up your system and install the AMD Dual Core Driver and Optimizer
---
Here is how I got my x2 3800+ to 2.5
Base Frequency (FSB) set to 250
CPU core voltage set to .13875
Multiplier at 10x (default)
Memory set to 166 (ddr 333)
Hypertransport (HTT) at 4x
Lock PCI at 33mhz and PCIe at 100mhz
Set SATA controller to 'Down Spread'
This will net you the following:
2.5Ghz CPU clock speed
208mhz final memory speed (416 DDR)
1000mhz HTT
At these settings you will outrun a stock x2 4800+
You can go higher than this, but [edit] after 210 actual (DDR420) you will need to start playing hit/miss with the memory voltages which may result in your system not POSTing. So feel free to aim for the moon, just be prepared to reset your BIOS at some point. [edit]