A celeron is an Intel, the brand that made Pentiums
For the motherboard if you don't know the brand and code of that, download this program:
Its a nice little program and will tell you all about your computer. From there, we can see if your motherboard is meant to overclock, and how good the feature is. The motherboard you see is also a worthy asset to overlcoking since it can have safety features and the like, and of course because it carries all the bus information, it is very valuable for overclocking.
As for your different brands of RAM modules, you might find this will cause a problem while overclocking, as each one will have different latency timings (Kingston being better I think). This might affect the performance of overclock you can get before your RAM would become unstable and crash the PC.
You should be able to get at least some, though I don't know how good of one... I've never really tried overclocking you see.
They do say though you should stick to the same brands while buying ram
Celeron D is actually a new processor, and is a good little performer, though I'm trying to research for you if the multipler is locked. I have a horrible feeling it is, which would mean you would only have the Front Side Bus (FSB), and voltages to mess with (Don't know about other things), though if it did have an un-locked multipler, of course you could push it harder, but you probably will find the RAM will push you back quite a bit
When you do get it sorted though, rise up by about 5mhz increments, and don't try to jump it to one straight away. That is def the best way of doing it, as you can see where it starts becoming unstable, etc.
I hope this helps