Kurtis - thanks for stepping in, it's helpful to have some comment from someone else who's played/plays WoW.
A graphics card will require a new power supply anyway, or at least I'd highly recommend one.
The Core-based Celerons are pretty decent actually for the price, I've built a few systems round them and found them excellent, the only disadvantage being that the 4 series are only single core (hence the newer E1200 and upwards series).
It is ok
Yes that card will want a newer power supply if it is under 500W for optimal use.
I admit, I have not had much experience with the newer celerons, only the older ones, up to skt 478, but only the ones before celeron D, which only had 128KB of l2 cache.
In comparison, my old 2.5Ghz celeron is far outstripped by the 2200+ @ 1.8 GHz, even in games, using the exact same graphics card (Radeon 9700 Pro)
That being said, your celeron 420 is of a totally different series and timeframe, so is alot more efficient at it's tasks.
I also noticed you were running Windows Vista too, this can degrade performance with poorly written drivers, and also with memory usage/page file, as you only have 512MB Ram.
Both the graphics cards you and Calum have posted are overkill for getting 60FPS in game, i would expect over 100+ with either card & 2GB of ram, with the main bottleneck being the CPU.