This will force people to come over to your website which should lead to more capture of leads. 

Business is all lead generation lead capture and lead fulfillment. 

@Lennie HollandI have a partner and we split everything right down the middle, we cn spend far more than that,,,, but it pays off, So Cal is a tricky market, we do a good amount of deals a year with decent spreads, but theres always those huge deals, that come around 2 or 3 times a year...when they come we dont mess around we put alot back into the marketing side of the business which can carry our marketing for months...and im no expert, but i bleed marketing, i love it, its steroids to me, so i study it, i study the big companies like Sears, Walmart, kmart...etc...and i see how they operate and i apply it to real estate...My partner is a sales guy, he bleeds sales...So combine we make a good team, and one thing we both learned coming from the mortgage world, is LEAD CAPTURING, so we understand that and it allows to keep track of everyone who calls, everyone we speak to, follow ups etc...And no matter what anyone tells you, direct marketing is always the best, a simple letter or postcard, wording is not very important just get to the point...The big companies survive from this, they know what you want before you even know you want it, they track every dime you spend, every keystroke you make, and they feed you advertisment all day, and you dont even realize it...how does the mortgage guy know you need to refinance, how does Sears know you bought a house and wants to give you 50% off all appliances, how does the car guy know you only have 1 year left on your 5 yr car loan, and hes offering you the best eal ever...THEY ALWAYS KNOW...and we as REI should always know...its the same concept, its just finding those people....But anyhoo i know you didnt ask for ll of this, its late, and i always ramble when its late....its a bad habit for me

All the website to do all the work while you capture leads all on autopilot.After that you can use those leads as you please. 

All you need is a website or just a simple landing page with a form to capture the seller info.  

I don't know your background so can't comment on whether this captures it or not, but here are a few design pointers.The design needs to be used strategically to draw the eye to what is important.

What capture system should I implement?

Have a capture system and a presentation and buy a bucket load of houses.  

Would Google Voice be useful as a “lead capture” system?

Most look at the .35 cents or so and think great price where in reality they shouldn't look at any number other than that of conversion rates, ROI and more importantly the cost of the infrastructure for the capture and fulfillment systems of the mail types.In short if I were only sending a few thousand pieces on a rotation I would send Yellow Letters followed by a Zip Letter/Check and then a postcard.