What exactly do you mean by 'adding a load of software'? Patches and add-ons to the game or something else?
It will make plenty of difference, for a start the 4650 is less than half as fast as a HD4850!
Do not be fooled into thinking the extra memory will make the card faster, in most cases it will only add a few FPS.
FSX is heavily dependant on the CPU and prefers a dual core to single-Exactly which processor do you have?
For a graphics upgrade you need to look to the HD4890 or GTX275, anything else is just too small an improvement to warrant the cost.
Check the game settings: AA is a particularly common cause of low framerates, either turn it down or off and you will see an instant improvement.
Fifty quid, you tight git;)
Forcing FSX onto one core would slow it down even more.
Look into overclocking the CPU, and card and check what memory you have installed, AMD chips like fast memory.
You COULD change the CPU, but the AM2 motherboard will heavily hamper the memory performance (Hypertransport AMD calls it), so to get the full benefits a new AM3 compatable MB would be needed, probably not worth it.
What resoloution are you playing in? Below 16x10 the 512Mb memory on the HD4850 is enough anyway, unless you have AA turned too high.
You might want to look at a 1Gb HD4870, I've seen some at 110 pounds UK.
Ha ha, indeed!! Unfortunately I am finding this 'hobby' to be more and more of a money pit, and having just shelled out a fortune on a load of new hardware, the PC budget is tight. I was looking to stay under £100 for a card as it is a temporary fix.
Can't remember what resolution I am running off the top of my head, probably quite high as the picture is running through a projector.
Is the 4870 actually likely to show any improvement? I am not sure where to go from here, I need to wait for the next gen of CPU's to start becoming readily available and just put up with what I have got for the time being.
I would suggest a little overclocking first, but read up on it! It is not the Dark Art some might lead you to think but it does have risks for the unwary.
Also, check through the game settings carefully, lowering a few details can have a considerable benefit.
As an example the game F.E.A.R. had an option 'soft shadows' that could halve the framerate while adding 4X AA to Crysis could do the same for little improvement in the image quality.