@flipcollar is very knowledgeable and from personal experience; I've also found this statement to be true.
My natural levels were unknowingly at ~80 (age 29) and sub-300 (24). Doctors always informing me that these levels were appropriate and just tossed SSRI's my way instead. After finally resting my case that I want to try TRT (after researching T-Nation TRT), I've prescribed only 125mg testosterone per week. I've been on it approximately 4 months now and have been making gains in size (mass, and no longer just fat), strength, and added endurance each week that I've never been able to accomplish under any circumstance (diet, training program, etc.).
So, when I read that people say that gains are impossible on anything less than 300-500mg/wk, I find it BS. It's up to your body. I think my body would be just fine on a 300mg first cycle and I would see some pretty decent results.
Another point is that people who B&C (blast and cruise) typically go for 250mg a week to maintain the mass that they gained (so I've read). So that only makes me question, why wouldn't it add mass to someone without a large stature already? You probably wouldn't see bodybuilder status mass gains, but, if like flip said, if your test levels are low, you will get size (and hopefully keep it).
I would definitely add an AI to your cycle though. I gained about 10 pounds of 'water weight' over a month on just 125mg.
Get your baseline (test levels) and if they're low, run it at 250mg in my opinion. If they're mildly-high, you may want to invest in running 500mg.