First off, I want you to know that graphic dedicated memory has less of a increase in performance (w/ increase of vRam), to the performance of the GPU, namely the # of shader units, SPs, the width of the memory bus, and the speed of the ram, and ofc OTHERs Many parts of the GPU dictate its performance, like a PC. Not only does the CPU dictate its speed, but the HD, the RAM, the GPU, and the NB, SB where applicable. GDDR3 and GDDR5 make a whole big of a difference for example; and we see this in the 4850s and the 4870s. The difference is 24%, with the rest of the GPU the same.
Here is something for you to think about. Where 2 256 mb 7300GS in SLI can theoretically run 512 mb of textures, and etc, one 8800GT with 256 mb can destroy the 7300GS to the point where it doesn't know if it is a nVidia card, and ATi card, or a 3dFX card
It is not about memory. Where a weak 256mb video card can theoretically handle 256mb of textures, if the core is too weak, it won't be able to handle the textures at the frame rate. Rather than spending more money on a crappy card, you may add maybe 10$ (maybe a card you are talking about is a low - end 6200, as such (OEM cards :shy
), you may spend the same money (as finding these cards may cost you $50 anyways), you can just go and buy yourself an 8800GS, which I find rather cheap these days. They perform at a 8800GTS level, which isn't that good, but still kills current day entry - card and low - mid end card. THis is given that you are still running a PCIe card currently. Given that your PC is an HP Factory Built PC, chances are you won't have a second PCIe port, unless you bought one of the highest end ones. But if you were, you probably won't need a new GPU anyways.
If you did have 2 PCIe Ports, you'd need to check if the mobo was SLI compliant. If it is, then you'd need to check you PSU. There are loads of things to check.
Best thing to do
, is to just get a single powerful GPU solution. They beat out multi - GPU solutions (in the mainstream, budget, and low enthusiast level) , and are a lot simpler to handle, usually (mostly) more power efficient, and depending on lifestyle level, more powerful too.
If you are on the AGP port, or even still on PCI, there are still many good GPUs to choose from. But at this point, if you had such, your best bet would be to simply get a new PC.
EDIT: It would be way easier if you gave us a bit more information such as a detailed spec of your current PC.
The whole story of PSUs. Again, many factors come into play. Crappy 500 Watt PSUs cannot handle much, where a quality built 350 Watt PSU can do a lot. If you are SLI'ing, then you might as well buy a new PSU, possibly a quality 500 Watt, or even 600 to be on the safe - side (depending on what GPUs you want to SLI).