I bought a 2010 Mac Mini about 6 months ago and I love it. First Mac I ever bought. I bought the entry-level 2.4GHz Core 2 Duo, 2GB RAM, 320GB 5400rpm HD. It ran sufficiently until I really started using multiple things on my Mini every day.
I upgraded the RAM to 8GB, and there has been no issue since. I recently added a 60GB OWC Mercury Extreme Pro 3G SSD and put it in my machine, and everything is running damn saucy. It almost as if I got a new processor. The SSD is the greatest improvement you can make to the machine, no doubt. It boots in 14-17 seconds with all apps open and running (via startup items), and shuts down in 2.
Though the 60GB SSD is a small amount of storage, all it has is Mac OS 10.6, and my personal documents. All of my Movies, Music, Podcasts and Mobile Apps (for iPhone and iPad) are on my Server.
But more importantly, your decision. I would expect a very minimal Mac Mini upgrade this year. Maybe an i3 2.0+ GHz processor, same RAM, and same HD options.
Personally, I would buy it now, and upgrade all the parts you can. Max out the RAM to 8GB, and replace the HD with an SSD.
Here are the links:
8GB RAM Upgrade:
http://eshop.macsales.com/item/Other World Computing/8566DDR3S8GP/
Mercury Extreme Pro 3G SSD:
http://eshop.macsales.com/shop/internal_storage/Mercury_Extreme_SSD_Sandforce/Solid_State_Pro
I hope my response helps you in your decision. Happy 4th!
-Alex