I have Celeron 1.7GHz 128kB cache, 128 MB RAM, D845GVAD2 MB
If increasing the RAM to 512MB is going to meet my requirements (internet, MSoffice, Video player, some anti virus SW) (with 128MB RAM, it is very slow) I will do that
Otherwise, Buying the second config - will it help ?
Or I should buy core 2 based system ?
Performance wise, about the same. Power consumption, obviously in the hands of the atom.
For the jobs you listed, the Atom would be better than the Celeron in a SFF design case. The Celeron 1.7 is a Willy, which are hot running archs. However, the better choice would be a new Sempron 140 along with a nice low power IGP mATX mobo, such as a 740G based unit. Overall idle system power consumption should be within a stones throw of a prebuilt mini ITX atom unit, while severely outperforming it under load, with power consumption easily less than the Celeron 1.7.
Case in point, my HTPC, before being an HTPC, was a simple daily rig, with a 65nm Sempron and Nvidia 6100 IGP mobo. Power consumption was just 37 watts idle, and an equally measly 52 watts load, and that rig would run circles around an Atom based computer! Best of all, I wasn't using any high end, high efficiency PSU, just a quality Rosewill 200 watt micro ATX conventional unit. With a power brick and pico PSU, power consumption would have surely dropped 10 to 15 watts off the top of the numbers. The new 45nm Sempron 140 is even MORE efficient than my older 65nm cpu. Ponder that fact for a moment.