Memory - Unless you go with 64 bit operating system, ONLY about 1/2 gig of that 4th gig will be usable. ie w/3 gig you have about 3 gigs available for OP and programs. W/4 gig ram you will have approx 3.5 gig available (or less) for op system and programs. This is due to memory mapping with a 32 bit operating system.
Processor, I would go with the T6400. The T6400 is a newer proc and more efficient Processor. Although again I dought you will notice any performance diff on most tasks. But there may be a small improvement for newer programs and there may be a small advantage in terms of power consumption and, or Heat disipation - for Only $25 I'd go with the newer Proc.