Your concern is valid, but there is no easy answer.

First and foremost, lag spike is a very vague term.
Getting 50-60 FPS in one area and then dipping to 30fps in a busy city with a lot going on is not really lag. That happens.... in most any game... there is a different between Max FPS, MIN FPS and AVG FPS....
In some games, the min/max difference is so high, they have to hard limit the max (Some as low as 30fps) to minimize the variance.

Some times it's a matter of game settings. For Instance, You get 40+ frames in one area... so you think: Cool... I'm good with 8xaa at 1920x1080 with the highest texture detail.... But if your min frame rate is 15fps in crowded areas, then yes, you need to drop those settings

The fact is... with crossfire ATI Radeon HD 6990s (I know, I know, crossfire support, blah blah blah) you can run on the lowest possible settings and resolution at a constant 60fps....
to take it a step further... with the same setup (And a quad core CPU/4gb+ RAM)), you can run at max settings (Except possibly general draw quality) with no area dropping below 40fps

With a GTX 260, dual core CPU, 3.25gb RAM, you can run on the lowest settings possible with at least 30fps in all areas...

So really just find out where you fit in there