3870x2 + 3870 Trifire?

Right now I have a single 3870 (Diamond Viper) and am wondering if any here have used a regular 3870 with a 3870x2 in trifire. I am only looking for a cheap way to get high fill rates (pixel and texture) as for shader my needs are simple and on a limited budget. No I do not want any of the 57x0 series and can't afford any thing over $100. Note I may not be able to purchase till later in the year.

Games played on this machine

Wow (up to 4 instances at once on 4 accounts)
Oblivion
Rise of nations gold
Age of empires 3
Second life (rarely)
Mech warrior 4

I am getting more games soon for this rig.

Cpu: 8250e (65w) oc 2.5ghz (88w)
Board: Ga-ma 790xds4 (got it cheap)
psu: antec neo He 550 (overvolts on 12v rails)
Case: don't ask its roomy and ugly