I was pretty much Hooked on GTX 460, so thats disappointing for me. Is there any way to remedy the situation like buying a new Dell casing that supports dual slot. Also can u give me ur take on the HD 6850. i have never used the Ati brand so im a bit affraid to experiment?
I've used ATI cards for years, ATI is currently owned by AMD. I once used nvidia cards as well although since the whole geforce 8400gs/8600gt overheating fiasco I have not purchased an nividia card. It appears that nvidia has corrected these issues and I would consider a card from them on my next build if their cards are competitive price/performance wise at that time. I currently have a Radeon HD5870 in my main system (no longer made but equivelent to a HD6950 performance wise) and it has been excelent performance wise. The 6850 is verry similar performance wise to the GTX 460
here is a comparison (this is a comparison of last year's cards but the 6850 is roughly equivelent to the HD5830) The HD5830 actually scores better than the gtx 460 in some benchmarks. The ATI HD 6850 has a slightly higher core clock and nearly three times the cores as the GTX 460 but the GTX 460 has a much higher memory clock. These more or less balance each other out making the HD6850 slightly better at some aplications and the GTX 460 slightly better at others. However due to this higher clock the GTX 460 runs much hotter and is far less efficent then the HD 6850 (This is why the GTX 460 is not avalible in single slot form).
Getting another case would be verry dificult today since the BTX formfactor is all but dead. Even if you could find a case The slot is likely too close to the motherboard IO pannel to fit another slot blank in between the PCI-express slot and the IO pannel.
Depending on the specs of your optiplex 755 you may want to consider building a new system. If you only have a Pentium dual core or 2mb L2 cache conroe or allendale Core 2 Duo (E4300,E4400,E4500,E4600,E6300,E6400) you can build a new system with a socket 1155 Pentium dual core or an AMD Phenom X2 for less than $200. (CPU, RAM, case, motherboard, you can reuse your current HDD and optical drive or replace them for another $60 or so) which will be more powerful than your current system. You will save about $100 on the video card by being able to use the cheaper dual slot version, so you end up with a new more powerful system for only $100 more than the upgrade would cost you. If you have a Core 2 Quad or 4mb L2 cache Core 2 Duo (E6550,E6600,E6700,E6750,E6850,etc.) It is likely worthwile to upgrade your current system.
megaman 598 may have a point on the PSU. Years ago OCZ was a good brand for the money but I haven't used one in a while so I'm not sure what they are like these days.