Mixing ram is ok, unless you hit some compatibilty issues, it'll default to the lowest speed and timings. Quite honestly, if it were Me, I'd flog the existing ram and get something like the OCUK value ram for £105 inc vat + del. That way you're future proofing the system, getting VERY good value for money, and all the timings and speed are identical, so you should be able run something like 2.5-3-3-7-1t @ 200. In this day and age, quantity (at the lower cost end) is better than quality (high speed, 230mhz and higher).
On the other hand, you could go for somemore expensive ram, say the ocz or gskill 'frinstance and over clock a fair bit with reasonable timings, but I reckon you'll only see the results in benching. In gaming, surfing, I doubt very much you'll see any tangible increase.
With 2gig, you most deffo will btw, games like FEAR, BF2 and COD2 will run a lot smoother, and prog and applications will open much quicker.