How much speed can I buy
A bicycle in and of itself is a half of the equation.  If you want to eek out as much speed as you can you also need to consider how you interact with the machine.  This answer focuses on the human/bike interaction rather tweaking bike specs.

Bike fit is a continual compromise between power, comfort, and aerodynamics (Bike Fit by Phil Burke). Simply plugging specs into a worksheet will not give you insight into these types of compromises.  New high-end bikes can be problematic as they are often very aggressive.  As you get older if you compromise the comfort dimension too much your power can drop of substantially.  You may be more aerodynamic, but you can end up slower due to a drop in sustainable power! 

Flat Land Speed

In terms of absolute speed on flat cutting down your frontal area (i.e., aerodynamic drag) will go a long way to increasing your speed. There are lots of little tweaks you can do to your current setup if it is currently fitting you well.


Tighter fitting clothing (any flapping clothing = aerodynamic resistance).
TT bars are another big aerodynamic gain (don't use on a group ride though).
Supple fast rolling tires pumped to the appropriate pressure.  (This isn't always the max pressure and can change from tire-to-tire.  You really need to experiment here)
Tweaking your current position to further optimize the aero/power/comfort relationship.
Drive train gearing


Tighter  gearing ratios (aka a smaller range in the rear cassette). This can let you fine tune your cadence more.
Big/big combinations are more efficient that Big/small gearing combinations for a given chain line.

Maintaining your drive train!


Climbing

Here a lighter bike will help, but also more appropriate gearing is very important (aka low enough to spin up all the climbs).  As Andy256 suggested you an play with some calculators.  But again I will harp on bike fit. Do you have the hand positions available to put you in a open upright climbing position so you can breath properly? Are your shoulders, neck and back relaxed on climbs.  Do you have enough hand positions (i.e.,drops) so that you can comfortably get in the drops on decent and faster sections.

Going up to get down

One day I finally admitted to myself that I could not ride the slammed positions of my youth and revised my cockpit putting myself in a more upright position.  The result of this is that I am much more comfortable on the hoods and can crank out the climbs at a harder pace.  I can also use my drops a lot more now as the higher hood position this give my back a change of position it now needs. My drop position is still quite aerodynamic position and I can spend a lot more time in the drop position than I have been able to in the past.  Overall I don't think my aerodynamics has been too adversely affected as I am using the drops a lot more than even when I was younger (then I used to just use them in crits, sprints and pursuits).  I wish I had taken more measurements when I was younger to confirm or refute this thought.

I suggest taking time to investigate how bike your current fit compliments your bio-mechanics before delving into the quest for new gear.  You might be surprised how many gains you can find!