I know this is what you may have already Google'd and found on your own, but here is my opinion, for what it's worth.
Grub v Grub2
I've had some older systems I upgraded and some fresh installs. Grub2 was developed with new technology in mind, so it performs the same function slightly faster and more efficient and will likely work better with newer machines, but you're not as risk if Grub is working for you as is. If you're doing a fresh installation, it's kind of a no brainer to go with what is provided.
On ext3 v ext4
ext4 gives you the capability to use high capacity storage devices and better performance based on how it organizes data, so if you're using application with lots of read/writes like a website that performs a search or some other transaction, you may have some benefits there. But, if you have less than a terabyte of storage and you're just you on the desktop in normal usage patterns, I would say there isn't a huge need for you to upgrade and risk losing anything (data or hardware).
On the kernel (http://www.h-online.com/open/features/What-s-new-in-Linux-2-6-36-1103009.html), I know there are some software packages that are slow to upgrade and aren't getting along with the latest version. You are getting additional support for hardware that was not available previously, so if you've got nVidia it'll likely improve what you're seeing.
So, if you're system is running sufficient for what you're using it for, then go for it. The performance increase you would see by upgrading I believe would be minimal, at least for the hassle of upgrading and troubleshooting. BUT if you go with a fresh installation, it would be a hard sell to me to do anything other than the latest offerings.