Depends on a few things, how much money have you got to spend? Are you happy with what you have at the moment?
The only thing that would make me start over if i was you, was if the mobo only supported dual core, and you wanted quad core, things like that. If you're motherboard supports all the upgrades you wanna make on it, then you should be fine with saving your money and just upgrading.
Just had a look at your motherboard, it supports up to dual core, and only up to 800Mhz RAM, you've pretty much got the best CPU and RAM you can put in there.
Rather than upgrading, i think the best option if you want a better PC, would be to get a new mobo, get something that can take quad core, faster ram, and possibly PCI Express 2.0. Then you have a wide choice of hardware you can choose for your CPU, RAM, and GFX card. A lot of people say quad core is not necessary, and most programs/games are not optimized for quadcore anyway, but it's my belief to be forward thinking. I'm sure they will start coming out with optimized games, then who will be laughing?
As for Intel vs NVidia, i would go for Intel. Most probably personal preference, but in general the build quality of Intel motherboards are much better than NVidia ones. GFX card, i would go for ATI rather than NVidia, because of the price/performance ratio is dominated by ATI at the moment.
As for specifics, i need to know what type of PC you're looking for, and what your budget's like.