Hmm, well its on a tight budget, but if you are gaming in any way I would say go with the E5200 and then get either of these bargains as a GPU - even if you don't game first person shooters or anything else like that, you will still notice a much bigger difference than between the E7400 and E5200:
The 4830 is probably somewhere along the lines of 3-4 times as powerful as the 9500GT - it will play every game out there at playable rate (not max settings though), which I don't think can be said to be in any equal measure for the 9500GT - at least in terms of games like Empire Total War, World at War, COD series, Crysis, WoW etc etc are all going to be substantially better.
As for the case and the PSU I will check that in a sec.
However if you go with a 400W power supply I would recommend going for the 4670 GPU instead of the 4830 which may draw to much power
Can't really vouch for the PSU on these - but looks like some people have had them for 5 years and longer -so that at least is a good sign.
I also think they will be better than what you chose to start with and you don't need a 500W PSU necessarily.
If you can spend a bit more money I would recommend you to spend it on the case and PSU - because if you buy good ones that you like you can use it for your future build. Read this for more on what components are important to spend money on: http://www.tomshardware.com/forum/264181-31-best-bang-b...
And this is a good list (really a must read I think if you are building a pc) of recommended parts for cases and PSUs: (also explains why more expensive PSUs and case really really are worth it - you don't want to fry all your equipment for example)
Geforce cards are not good value sadly nowdays (in the budget range that is) and ATI is fine, but if you have to go with NVidia then I would try to get at least a 9600GT, which is a solid card (still not as good as the 4670 or 4830) or if you can spend the money then the GTS 250 is excellent - that is the first price point where NVidia is competitive (at least in the US) IMHO
-edit, just saw the last comment - still think you will benefit much more from the E5200 and a better video card. The E5200 runs every day to day program and multi-tasking with the same ease as the E7400, the only thing better for multi-tasking is more cores and the only thing you will ever probably notice if you did professional bench marks between the two in the programs that you use on a daily basis is that when you are encoding mp3's, defraging your drives or playing games the E7400 will be a bit (just a bit) faster. You won't notice this in real life because the difference will be that little (pretty much academic). For the light use you describe the E7400 is pretty much overkill, the extra MB of cache and clock speed is something you will really only notice if you run CPU heavy programs (like professional applications that need to compile or calculate vast amounts of mathematical data or games that do the same).
Also mate - what series was that ATI card? was it back in the days of the X1000 series? I had one of them, an X1600 mobility in a laptop which ran way too hot and crashed a lot. ATI has picked up their game a lot since then, but I would look through the forums and see what people say about the stability of any card before you buy it, also check if it has driver issues - that can be a problem with ATI sometimes - but again they are much better there now as well.
yeah they are about the same performance wise - so really no reason at all to get the 9500GT - you could just use the old card and it would be the same. Also the chart on this page should give you a relative idea of the performance of the different cards