Hello Z, I have been working with the m55-sli board for a while so I think I can help you to get started. This board is a good start into OC'ing but it has limited features compared to more expensive boards from ASUS or ASrock. My setup is very similar to you. I have the board and a x2 3500 Brisbane cpu. Let me give you the two most important things to remember.
1. The m55sli board has 2 OC menus in the bios. First is the MIT menu where you can adjust voltages, cpu frequency, etc. econd is the advanced chipset features menu; press Ctrl + F1 at the main bios screen.
Also remember to disable the AMD cool and quiet option.
2. This board does not have advanced memory timing options so forget what people are telling you about changing memory timings, etc.
Now here is a basic setup plan for the board.
1. In the advanced chipset features menu set the HT link to 4x or even 3x. If you want to push the cpu past 260Mhz you must do this first.
Then go down to the DRAM config menu and select manual and set the frequency to 400. This will give you some headroom to OC your cpu.
Now you will see the extent of the memory options for this board. Not much! You only have a choice to set CAS latency to 4 or 5.
Now a point about your memory. It sound like you have budget Ram, just like I did. This is not good enough for a cpu like this. You will soon find the need for better performing DDR2 800.
2. At this point you are ready to adjust your cpu. I guess you have seen in the MIT menu of the bios there are some basic features for this.
Honestly at this point you should consider the EasyTune 5 program to start tweaking your cpu's FSB speed. Start tuning usng the stock CPU voltage and multiplier. Increase your FSB speed until you reach 260MHz, then you will want to start checking your temps and your memory frequency. Chances are your Ram will not exceed 530Mhz so this will eventually put a cap on your cpu speed maybe around 270Mhz. As I said before there are no advanced memory options, this board basically uses the Ram's default timings. No problem just save for better Ram, as well as a better psu.
I believe you will see that although this board has some limitations it is still very effective for beginners.
x2 3500+ Brisbane
GA-m55sli-s4
Seasonic M12
G.Skill PC6400 2G 4-4-3-5
XFX 7300gt x2