To what extent does the memory affect bitcoin performance?

Specifically, I am filling a PCI Express x 16 slot with the best single-width card I can find (double slot cards would crowd the next GPU and cause heating issues)

The following are two cards from the same mfgr, and which only differ in memory type (and $10 in price)

For this type of work, will one memory type or another make a significant difference?

PowerColor AX7750 1GBK3-H Radeon HD 7750 1GB 128-bit DDR3
http://www.newegg.com/Product/Product.aspx?Item=N82E16814131481

PowerColor AX7750 1GBD5-DH Radeon HD 7750 1GB 128-bit GDDR5
http://www.newegg.com/Product/Product.aspx?Item=N82E16814131461

The process I run generates btc, but not for *me* - I do get in-game currency for an online game I play (Wurm Online) for crunching at night, and I have another PC running a 7770 and what it does (about 200 Mh/s) generates an amount of in-game currency that is worth my time directing the PC to that task.  I also run my main PC and it's 7970 at night or when I am at work.  I was simply looking for a way to get the 3rd , much older, PC into the pool using the AGP 4670 card.

I had looked over that chart before, hence the aim for using the 4670, since it does do the work, above comments notwithstanding; it was the best available AGP card I could readily find.

Processing rate is not a concern.

Exactly what generations of Radeon cards *do* support the work?

 I have multiple 7000-series cards working fine on other PC's.  I'm trying to get this older PC into the mix.

Will a 5450 on the regular PCI slot do the trick?

Trying to get an older system to work properly with OpenCL.  Several potential obstacles and looking for configuration solution or cheapest upgrade path:

Tyan Tiger S2466N  motherboard
AMD Athlon MP 2400 x 2
2 GB ECC memory
HIS 4670 (AMD/ATi) AGP video card w/1gb ram
Windows XP Pro

The closest I have gotten is using the original HIS drivers in conjunction with a somewhat older AMD APP SDK that includes the OpenCL runtime.
It says I can use the CPU's for OpenCL work, but that the driver doesn't support OpenCL.

AMD has dropped OpenCL support from current XP drivers for this range of video cards, though some older ones that might still support it can be dug up.
Also am concerned if my using Athlon MP's is a roadblock, as they lack SSE2 support

Perhaps someone can point to a particular combination of Drivers/OpenCL SDK that gets along together and can let bitcoin apps use the video card?