I had written a big shpiel on this the last time it came up, but here's your options for telephotos.
Minolta 70-210 f/4 (old lens), colloquially referred to as beercan. All-metal construction, so-so AF
, but has a unique image quality. If you find a good copy, hang on to it! Only found used.
Minolta Original 75-300 f/4-5.6 - colloquially referred to as the Big Beercan. Looks just like the original Beercan, except it extends when zooming and has a focus limiter switch. Only found used.
Minolta 100-300 APO - Avoid non-APO versions of this lens. Can be had relatively cheaply these days as the 70-300 G has depressed prices for used copies. Only found used.
Minolta 100-400 APO - Similar build/IQ to 100-300, except it goes out to 400mm. 400mm is very slow at f/6.3 though. Only found used.
Tamron 70-200 f/2.8 - Tamron's 70-200 f/2.8. Has great image quality but it's screw drive and not silent like SSM options.
Sigma 70-200 f/2.8 HSM - Sigma's version of the venerable 70-200. Has HSM on Sony. Costs a lot less than the Sony SSM lens.
Sony 70-300G SSM - higher quality telephoto from Sony. Has SSM focus, great image quality wide open.
Minolta 80-200 APO - This lens was replaced by the SSM version. Still pretty good but can only be found used. Screw drive focus so it's not silent. Only found used.
Sigma 50-500 - Not HSM for Sony, in my opinion outclassed by the 70-400G.
Sony/Minolta 70-200G f/2.8 SSM - Sony's standard 70-200 f/2.8 zoom.
Sony 70-400G SSM - Big silver lens, brand new, expensive ($1500). Awesome IQ, though.
Minolta 200/300/400/600mm primes - Excellent image quality but can only be found used.
Sony 300mm f/2.8 - Stupidly expensive but awesome image quality. Built like a tank (and weighs like one too).
If you're looking for a wider lens (a replacement for your 18-70) consider the Tamron 17-50 or the Carl Zeiss 16-80. The Sony 16-105 is good but the CZ
is sharper. There's also old Minoltas like the 28-135 (a true classic) and other 24/28mm starting points but they aren't very wide on crop bodies like the a200.
For a beginner, look for a beercan/big beercan on eBay. If you want a new lens, start looking at the entry level premium options, like the Sig/Tam 70-200 f/2.8s or the 70-300G. Consider reading Dyxum's lens database for reviews. http://www.dyxum.com/lenses/index.asp
The forum there can also give you some good advice.
[Edited 2009-06-01 09:39:44]