You should be able to upgrade your CPU but sometimes whether its the vendor or not the upgrade doesnt work even if its a compatible processor (I have done this in the past, most of the time it works if its a rather minor jump from processor to processor other times only a black screen appears after starting the laptop no matter the jump size). I see you can theoretically upgrade to an low wattage sandy bridge i3/i5/i7 processor (celeron to i7 is a huge jump, to i5 is a big jump, to i3 not much of a jump), which would update your graphics from HD Graphics to HD graphics 3000 as well. You can try out CPUs like: i3-2377M or an i5-2557M or i7-2637M
If your main purpose for this upgrade is gpu performance then I would tell you to look for a new laptop as the gpu is still weak, though on the cpu side its a rather large upgrade. Also as you know stick with only the 17W CPUs since your motherboard is designed for low power needs (which makes this upgrade even more riskier!)
Also though it can be a bit more risky updating your BIOS to the latest version can sometimes help too (but hey, everything in this is a bit risky so the more riskier the better)