Question: I am planning on getting a new CPU and am looking at a quad core i5.

The reason for the quad-core is that I generally multi-task a lot for example running Visual Studio, Eclipse Aptana, Photoshop, Lightroom and lots of Chrome browsers all at once.

1. I am wondering if it is possible to allocate certain processes such as Visual Studio permanently to use a particular core, and is there a tool better than Windows Task Manager to achieve this?
2. Will this give me a snappier multi-tasking experience or will I not notice the effects?
3. I am also planning on moving to 8GB from 3GB as I experience severe thrashing after a couple hours of web development with multiple IDE's open and using Chrome's developer tools. Will the additional RAM be enough, or do I need a faster CPU? (currently running Core 2 Duo E6750 @ 2.66ghz)

Answer: 
Assigning CPU affinities to specific executables is a bad idea. Setting affinity on a process doesn't reserve a CPU for that process you specify, locking out all other processes from that CPU. It just says that that process can only use the designated CPU. 

That's a problem because there are far more processes needing CPU time than the ones you're normally aware of. If you use affinities to bind a process to say, CPU 0, and something else is using CPU 0 when your process happens to need CPU, your process can't run at all until the something else is done with CPU 0. 

It's much better to run somewhere than nowhere, and running nowhere is a common result of setting affinities. 

Another horrible result of setting affinity on a process to a single CPU is that you confine all of that process's threads to the one CPU. Thereby killing any performance advantage that multithreading might have had. 

In short, just let the Windows scheduler do its thing. The fewer options you let it have, the poorer a job it can do.
Jamie is correct BUT, there are some mainstream programs that run much better on a single thread like AutoCAD and I do assign it to a single core and it runs faster with less lags. That being said there are other components of AutoCAd such as printing that will then seem a bit slower doing this as those processes may lend themselves to multi-core functionality. I am sorry to say that the software is not well optimized for multi-core systems still!!!