Your current system would be fine with a quality 350W PSU. The quality and amps on the 12V rails are more important for a modern system than the total maximum wattage. To get an idea of power requirements, have a look through the power draw measurements in TR's various CPU and GPU reviews. Note that these are always measures of power taken at the wall plug, so the system is actually using less because of PSU efficiency (e.g. if the system is using 240W with a 80% efficient PSU, it will draw 300W at the wall). You'll soon find that a system with a dual-core CPU and a single graphics card will struggle to use more than 300W. If you assume this is all needed on the 12V rail, then you can easily choose an appropriate PSU. It's only worth considering getting a more powerful PSU if you anticipate adding a whole pile of disk drives, or upgrading to a more demanding system (paricularly multi-GPU) in the future.
You could also look at TR's recent affordable PSU roundup here
. The system they used for testing is not a million miles away from yours, and all but the cheapo generic supply were more than comfortable.
Note there are also various online PSU calculators available (see this sticky
at the top of this forum), but in my experience they all tend to be rather conservative and over-spec the required PSU somewhat.