There are numerous factors to consider besides "speed & memory". Not sure what you mean by "speed".
There are different types of memory used on video cards...older cards used SDRAM, then moved on to DDR2. GDDR3 is currently the most common but some are beginning to use GDDR4. Plus there are different memory interfaces used...64-bit, 128-bit, 256-bit, etc. 64-bit is low end, low performance. Most mainstream gaming cards use 128-bit or 256-bit...high end cards are moving to 320-bit, 384-bit or 512-bit.
There are a LOT of other factors too...make/model of GPU, number of pipes, DX support, etc.
This guide from newegg may be helpful:
http://www.newegg.com/ProductSort/C...
And this site lists the specs of most of the cards made over the past few years:
http://www.techpowerup.com/gpudb/