There's absolutely nothing wrong with the 50mm you have. The f1.7 version is sharper at larger apertures but it isn't worth spending limited funds on changing, you can still take superb photos with the f2 version.
The main thing to bear in mind is that prices are very stable for all genuine SMC Pentax lenses (as they can also be used on their DSLRs), so you can safely buy, see what you think, and sell if it doesn't suit you. Try searching ebay daily with it set to show newly-listed items first, you can often pick up bargains on Buy It Now items listed cheaply.
In your position I would look at the 28mm f3.5 (there are SMC Pentax and SMC Pentax-M versions, the former is slightly sharper but bigger and harder to find, the latter is still pin-like but more common) and either the Pentax-M 135mm f3.5 or the Pentax-M 80-200mm f4.5 - try to get the first version of the zoom with the chrome ring between the grip and the lettering as it's reportedly better than the later version without this. All are easy to find and shouldn't cost much, if you later decide you want something more exotic (like the SMC Pentax 135mm f2.5) then you'll get back what you paid for them and can put it toward the new lens.
In general it's best to avoid Pentax lenses which don't have "SMC" in the name, those without were designed to be cheaper. They are coated but not SMC'd, and were frequently simplified - the 28-80mm zooms for example are best used for anything other than photography! Pentax also reused the Takumar name on some K mount lenses known as "Takumar Bayonet" - also best avoided for now, while a couple of them are decent enough you can do better for the same money or a little more. There are a handful of lenses with SMC which are still best avoided - the 35-80mm zooms and the 40-80mm zoom. The 35-70mm zooms are perfectly decent (I'd suggest the Pentax-A 35-70mm f4 as being the best fit for a K1000) but the 35-80mm in Pentax-A and Pentax-F guise is horrible.
Don't be caught out by the Takumar Bayonet 135mm f2.5 by the way - it's nothing like the proper Super Takumar 135mm f2.5 or the SMC Pentax version. Some unscrupulous sellers are listing the Takumar Bayonet with a high BIN price in the hope that people will mistake it for the more desirable lens.
Last edited by PentaxBronica; 09-30-2012 at 09:56 AM. Click to view previous post history.
Reason: missing "m" in 80mm