Is it worth spending $500 on a three-year-old computer that is out of warranty or will be out of warranty in four months? It
might be, but entire Apple notebooks are available for about twice that on the refurbished store.
You ought to consider whether you’re realistically likely to stick with your machine for another three years. USB 3.0, Thunderbolt, a quad-core CPU, and an IPS Retina display might seem pretty necessary for someone like you before then (which isn’t to say someone else wouldn’t still happily use your 2010 Mac – and they’ll pay you nicely for it on eBay).
Although SSDs are very fast, they’re not without their problems. I’ve heard of many problems with aftermarket SSDs. Theoretically they should be super-reliable and problem-free, but for some reason they’re not. (OEM models are much better.) On the other hand, your suggestion of an Intel SSD might minimise the risks.
But then you mention you’re nearly filling a 500 GB hard disk. I’d treat that as another disincentive to switching to SSD.
If I were you I might get the RAM (though it’s surprisingly pricey!), leave the hard disk alone (or spend $85 on a
1 TB Hitachi TravelStar 7K1000, which would give you some speed and capacity now, and would act as a nice scratch disk or backup disk for your next Mac), and get a new Mac sooner rather than later. Any expensive upgrades you make to your 2010 MacBook Pro will add little to its secondhand value.
Edit: sorry, I didn’t notice/absorb the “Early 2011” in your thread title. This changes things a bit, because you have Thunderbolt and possibly a quad-core CPU. Still no Retina or USB 3.0, of course. By the way, is it a 17-inch model? That might be worth heavily upgrading if you like it, since the era of 17-inch notebooks appears to be over – at least for now.