For the general use of home users, no.
Regular game players will need atleast 2g, going forward. 4g would simply negate any considerations that memory is holding u back at all (as long as the mobo can use it at the speeds required) - I do believe that a simple game like the football management types will run alot smoother, with all leagues used, @ 4g.
However, the likes of Excell being such a poorly written piece of software, I see it working alot better the more memory it has. Sound editing likes alot, understandably. I would say gfx editing too, cept most I`ve seen use Macs for that, with a fraction of the requirements.
Believe the ceiling for XP is 4g, don`t know about Vista. I`d`ve thought Vista would let u go up to 8g, I know some users who`d benefit from that. Does it ? I dunno.