Not sure what your supply situation is as to whether or not this idea would be viable but bundle deals are quite popular, especially among people who are having to upgrade for a new game coming out and find that their "old 775 board" or whatever no longer has an upgrade path due to being phased out.

As you likely are aware, most are:
mobo - ram - processor
some also include a cooler and what you can do to cross-sell to your other products is have something like a drop box at the bottom with

[add one of these graphics cards to your build]
Asus DirectCU2 GTX460 OC: £140
Asus GeForce GTX 570: £290 etc etc

You'll notice that I have taken your current prices and added a small discount which is the way to do this as this customer is buying almost a complete computer from you so the discounts are outweighed by the quantity they are purchasing

If you currently don't have the supply lines for it then it's definitely something to consider for the future as most companies buy-in preset bundles, you have the opportunity to do something different and create your own bundles from your experience of what works and so on.