The difference between a Willamette P4 and a Northwood P4 is actually pretty easy to see on the chip itself. On the chip itself, look at the very top line of letters/numbers. (The line written directly underneath Pentium 4.) If it is a Northwood CPU, the very last set of numbers on that line will be 1.5V. If it's a Willamette, it will be 1.75V. Also, on that same line, after the speed designation, there is the designation of the cache. For a Northwood, it will be 512. For a Willamette, it will be 256. So if your CPU has both 512 and 1.5V on that top line, then you have a Northwood. If those two sets of numbers don't exist, then you have a Willamette.
As for your RAM question, I'm not too sure. Just because it's OEM doesn't mean that it is of a lesser quality. If you knew which company actually produced the RAM chips, it'd be easier to tell. However, PC3000/PC3200 isn't really officially supported by very many P4 DDR chipsets, if at all. (Somebody please correct me if I'm wrong.) So basically, either one should run just fine in an overclocked environment. (Just avoid MSI motherboards because they give you jack-shit for memory ratios. :| My 1.8A@2.52 GHz CPU is being choked off by a measly 2.0 GB/s memory bandwidth, and there's not a damn thing I can do about it until MSI gives me A memory ratio for my 845 Ultra-ARU board.)