The only downside to the 945 is that it is a locked chip, say compared to the 955 BE which is unlocked giving you overclocking headroom. Only the added bolt on of L3 cache will make the Phenom II faster but the Athlon is rather close in performance so its not a huge upgrade but a upgrade nevertheless.
I wouldn't throw any more money on to that, I would rather save up to get a new Mobo, CPU and RAM down the line, I don't really see you getting to much performance benefits going to a 945, possibly at best around 5-8%.
Just to put it in context newegg has a special which involves a 8120+Biostar TP990FXE + 8GB Gskill Snipers + Roswill Hive 600w + Kingston SSDnow 120GB for $480+ CASE can't remember which one.
cheers for the reply mate im in the UK not really a case of needing to save up just wondering wether to justify the CPU upgrade, or as you say give it a year or so and just buy a new rig, to be fair its mainly just surfing and watching movies so it will do for now :-) only has a 4670 video card so its not a gaming setup but thanks for the views looks like little gained from the upgrade so will leave it :-)
Honestly save up the bucks and use them when it really matters when you want a full on upgrade, for surfing and the like you aren't going to get any benefits and in the end you are going to be left with all of this anyways.
Athlon II x4 640 is a nice chip though, you may want to hold on to that for nostalgia.