For a lot of the stuff you use I don't think it will make a huge difference (though at that phenomenal price, why not), other than keeping some more programs open.
However when using Photoshop (and Illustrator, etc), depending what I am doing I can easy burn though 4gigs, so yes, it is very noticeable and helpful there.
Also I think it would definitely give you a nice boost in the video encoding programs.
And the comment about the harddrive is also correct to I believe, having a slower 5400rpm drive (as I have too) will benefit more from the 4gigs, the extra memory will help prevent pageouts (writes to harddrive) and boost sys. proformance. Also if you have the iStat widget installed you can see your pagein/outs with you current RAM allotment, you should have a 2:1 ratio of ins to outs, (a higher ratio is better), a 1:1 ratio is not great, and a 1:anything over one is really bad. with 4gig ram I don't think I ever get over that 2:1 ratio
my .02