Please dont laugh too hard...
But my Gfx card is an Nvidia Gtx 260 and my CPU is an Intel Core 2 Quad Q6600
Ouch. Yeah, that's a little out of date. I can't find any benchmarks on Monero for the 260; I did find someone saying he's receiving 32.5 MH/s on Ethereum mining with a GTX 290, which seems great considering the rates more powerful graphics cards are getting. That's roughly the same hashrate I found on an old benchmark chart showing GPU efficiency in Bitcoin mining for the 260. Obviously 30 MH/s wouldn't be worth the power it costs with the current Bitcoin difficulty, but if it can really pull 20+ MH/s on Ethereum that's fairly solid, enough to mine 1.75 ETH a year at the current difficulty. The 260's reasonably low on electric costs at ~180 watts, which would put your profits at about $4-500 a year. If it performs similarly well with Monero, at a 500+ H/s rate and that power draw it would be about the same profit rate. Depends if those hashrates are accurate or not though, I'm curious if they are.
Ok, a few questions spring to mind from you post.
1) GUIMiner tells me my GTX 260 cant be used to mine with?
2) Which mining software would you recommend for casual/hobby mining of Eth/Mon on my GTX 260
3) I see lots of different variations of ETH... which is still GPU mining on a home PC?
1) I've never heard of GUIMiner, can't help you on that one. It seems like people used to mine with the 260 from the brief research I did on benchmarks, so maybe that particular software just isn't configured for it? If there's a way to contact the developers you could try that, or just try other mining software.
2) I haven't personally mined Ethereum, but a quick google search led me to this:
https://steemit.com/ethereum/@gobbahfett/easiest-guide-to-mining-ethereum-eth-windows-amd-nvidia-part-1. If it doesn't work, there are Ethereum-specific forums you could check on to get a much more comprehensive answer, I'm sure. For Monero, the most popular software is Xmr-Stak:
https://github.com/fireice-uk/xmr-stak-nvidia.
3) ETC is Ethereum Classic, which is not used all that much anymore. It still trades, but for a far lower rate than ETH. Most people are mining ETH over ETC because it's more popular, more expensive and therefore usually more profitable. As far as I know both versions are still using GPU mining, since Ethereum's blockchain is ASIC-resistant. Monero is as well, which is why those are the two coins I recommended. There may be other ASIC-resistant coins that I don't know about, of course, but ETH and XMR (Monero) are trading relatively high recently and should always be profitable with GPUs, though power costs may eat into that depending on your price per kWh and the efficiency of your GPU. If the benchmarks I saw for the GTX 260 are accurate, you shouldn't have to worry, but the best way to actually test your hash rate would be to install a miner and check the rate after a few hours.