Unfortunately PerfHUD isn't going to be of much use, since you have an ATI GPU. However they do have their own equivalent, called GPU PerfStudio. Either way these tools will only give you detailed statistics about your app's GPU performance characteristics, as well as your D3D driver performance. If your problems are elsewhere, you'll have to use other tools.

For checking your GC performance, you can just use the Performance Monitor that comes with Windows. Just open it up, and add some counters. There's a whole bunch of CLR counters, and the GC-related counters are in ".NET CLR Memory".

PIX can also be a useful tool for monitoring performance, since it can monitor various system and D3D-specific counters on a per-frame basis and also display them on a HUD over your app. It can tap into all of the system counters, so you can make a custom counterset that has the normal D3D stuff (Draw calls, SetRenderState calls, etc.) and also has some .NET performance counters. You can check out the tutorial in my sig if you haven't used PIX before.