The SSD will give you an overall performance boost, as the HDD is usually the slowest component. You can always add the RAM afterwards if you feel the need, and you can get faster RAM modules too.

Personally, since you're doing 3D and other relatively memory-intensive apps, I would upgrade your memory. I had 8GB of memory in my previous PC and it wasn't difficult to get memory usage up to 80+%. I was able to actually suck up all 8GB and then some in Photoshop when manipulating and combining multiple images. Recent versions of any decent 3D or imaging apps will be 64-bit to take advantage of memory. Not to mention, more memory = more cache. Upgrading to an SSD will help, but the OS and apps will also take advantage of memory for cache so you don't read frequently-accessed files/data from disk as often. And memory is cheap, upgrading from 8GB to 16GB should cost less than $50.

Only you will be able to know which will be the most help. Have you monitored your resource utilization at all? If not, you should and then you'll be able to tell how close you are to using your current 8GB and how busy your disk is while using your apps.

You can track memory, disk I/O and utilization, and a bunch of other metrics with Windows' built-in Resource Monitor.