"SSD are degraded with writes" is now mostly a myth*
Windows 7 has built-in TRIM support and most modern SSDs practically eliminate these early issues (3rd party software or internal work to clear/garbage-collect unused pages). Anandtech and Tom's Hardware have some great articles and write-ups on the subject. The core issue is that "clearing" a flash page is a relatively long operation; if this is done ahead of time or in the background there is no appreciable degradation (the memory itself does not get slower over time). TRIM lets the OS tell the drive when a page can be cleared/recycled -- as far as the SSD (or HDD) is concerned, a page consisting of nothing but zeros is still valid data.
Put everything you want fast, INCLUDING OS, SOFTWARE, AND SWAP, on the SSD. (This naturally extends to Visual Studio projects and pretty much anything else except for large media or archives :-)
I would recommend an SDD based on the SF-1200 (or higher) series ~ $200 or less for 120GB. Except for capacity and cost, there is no reason to ever go back to spindles -- a good SSD is so fast at random access, that even under moderate-heavy load swap is "almost unnoticeable" or "minimized to the point where it does not bother me". (An SSD is still significantly slower at random access and throughput then main memory, however. Being good at swap is a stop-gap and not a real solution for a demanding user.) In this aspect there is absolutely no contest between an SSD and HDD in terms in of random-access performance and latency: a good SSD will win hands-down in every test/scenario that favors this usage pattern and even a good HDD will make a system chug under moderate-heavy swap. With everything else equal, I would get a good SSD over another 4GB of memory, but best would be to get both ;-)
Data read sequentially, like movies or large archives, will see the least benefit moving from platters to flash chips (but will still see a benefit) -- once a HDD gets going it has a pretty high throughput. In this aspect the role of HDD's is relegated to the task of mass storage and not every minute usage.
I will never willingly not have an SSD as a primary drive again. Even a "degraded SSD" (say, 80% optimal) is still much faster than an a conventional HDD -- but remember, that just like a traditional HDD, a SSD will eventually get old (and "slow" in relation to current technology) and fail -- use it while you have it and keep external backups. Also, not all SSD's are equal. Again, my recommendation is an SF-1200 (or higher) based device -- it is just a "consistently good" prosumer controller for a competitive price (throughput isn't everything, consider average/max latencies, etc).
Now, get to a techie site, read a few articles, and put in an online order for a shiny new toy :-)
*If it really is a myth depends upon the actual SSD controller/firmware, operating system configuration/support, external maintenance tools, etc., of course. Windows 7 comes with TRIM support and the SandForce (e.g. SF-1200/1500/2xxx) controllers are TRIM-aware; they are also fairly aggressive at internal collection and page-cycling. With good controllers and TRIM support the statement "SSD are degraded with writes" really does become a myth.