Can you give a little more detail on the exact ENVY dv6 model you have?
I am in a similar situation, in that among other things, I was thinking of upgrading the graphics card. I was pretty sure it would be too costly based on several factors in the ENVY dv6 series (and HP in general), and my research has lead me to believe I was unfortunately correct in that assessment.
First, HP notebook motherboards (as well as a fairly large selection of notebook motherboards in general) tend to have their "discrete" graphics cards either soldered in to the motherboard, or actually integrated into the main board circuitry itself. Secondly, the ENVY dv6 6xxx and 7xxx series notebooks (and others) have locked BIOS chips with white list components which prevent the user from flashing BIOS or using any components which are not sold directly through HP at a very high markup.
So, depending on what model in the dv6 line you have, in all likelihood you will not be able to swap in a new graphics card by itself. What you can do, however, is replace the motherboard with one from the parts manual, and depending on what CPU you are running, you can get the correct board with the 650M chipset which has the nVidia GeForce GT 650M with either 1GB or 2GB GDDR5. Unfortunately you will not be able to get one of the higher performance GTX chips as they are not supported by the BIOS - the 650M is the fastest available.
What will you be using the graphics for, and how performance intensive are those tasks? I initially considered swapping in a new motherboard with the 650M and 2GB GDDR5 VRAM, but after opening some fairly GPU stressing programs/games/etc using the 630M, I was quite happy with the performance, benchmark values notwithstanding. I have found that while benchmark software provides a reasonably good theoretical comparison of performance, in real world situations you often get a lot more out of a given component than the benchmarks might suggest.
I have an ENVY dv6 72XX series notebook with the following:
Intel Core i7 3630QM (2.4GHz - 3.5GHz - 6MB L3 - 1600 MHz FSB) 4 Core / 8 Thread CPU (Stock)
Seagate Momentus XT 750GB* (8GB SSD / 750GB HDD) Hybrid Drive (Aftermarket upgrade from 750GB 5400rpm HDD)
nVidia GeForce GT 630M 2GB DDR3 VRAM switchable graphics (up to 4GB total with 2GB additional system shared SDRAM)
12GB DDR3 1600 MHz SDRAM System Memory (Upgraded from 6GB)
BD-RW / DL DVD+/-RW SM Drive (about to replace DL DVD+/-RW SM drive)
If you see what I have now compared to stock, only a few minor details have changed really. I added the Hybrid drive which adds an 8GB NAND flash memory SSD to the 750GB HDD to achieve ultra rapid transfer rates while still retaining the high storage capacity of a HDD. I doubled my SDRAM, but I really only ever got up to about 65% memory use max on only one occasion with the 6GB while trying to stress test the system, so I probably would have been fine staying with the 6 for now. Then I added BD support in my optical drive, which was just an optional addition since I came by a BD-RW drive for free my friend didn't want from a broken dv6.
The Intel Core i7 3630QM is an incredibly fast - blazing even - CPU, and sits at the extreme high end of the benchmark tables, outperformed by less than a half-dozen or so notebook CPUs, and only the very topmost performing percentage of extreme high end desktop / server CPUs. It is many times over again faster than the best that AMD has to offer ATM.
The nVidia GeForce GT 630M 2GB DDR3 actually does sit in the mid-performance ranges, but still outperforms the native Intel 4000 HD graphics in my system quite handily, and while it is among the better GPUs found in notebooks save for the 650M and up models, offers what I'd call high mid level performance with regard to all GPUs, and mid-high range performance when compared to 90% of notebook GPUs currently found in today's market by real world market share. I have gotten fine performance out of it at high detail levels with most graphics intensive apps/games I have run, having to turn down effects to medium detail on only one or two ultra high end high performance games I have. I don't have much time for games these days anyway, so I am more than happy with this level of performance.
If you do require ultra high end graphics, consider a GTX 660M with 2GB GDDR5 or a GTX 670M with 3GB GDDR5. You won't be able to put them in an HP, but they are actually somewhat easy (by comparison) to find in ASUS notebooks, especially in the G55VM and G75VM sieries ROG notebooks. A system with an Intel Core i7 3610QM CPU, 8-16GB DDR3 SDRAM, and a GTX 660M with 2GB GDDR5 or GTX 670M with 3GB GDDR5 can run anywhere between $600 and $1800 depending on if you get new/used and max out on things like multiple SSDs, BD drives, etc etc. There are ASUS Gx5VM series notebooks with 1TB HDD, 12GB DDR3 SDRAM, and nVidia GeForce GTX 660M 2GB GDDR5 GPUs for under $1000 brand new.