I have a Dell Inspiron desktop 546 (not the slim) that I am trying to upgrade. I purchased a Sapphire Radeon HD 6870 GPU and a new Thermaltake 500 W PSU to power it.
I removed the old PCIE Radeon HD 4000 series GPU from the mobo but when I went to seat the new 6870 in the [only] PCEI slot, an old standard PCI nearby was in the way (the 6870 has a large heatsink and fan attached that collides with the old, taller PCI slot)
I have searched for answers and the best suggestion I have seen is to use a PCIE riser or cable extension (I ordered a cable). I have also seen some somewhat questionable advice to cut the old PCI slot with an exacto or to attempt to desodder it.
My questions are:
 once the PCIE cable is delivered and installed, how am I going to support the card within the case? Without a firm PCIE slot to sit in, it won't be secured- especially since I anticipate needing to snip the expansion slot on the rear of the case to accommodate the relocated card...
 were I to purchase a new case designed to fit large GPU cards, like the Coolmaster Elite 430, I would be in the same boat... how do you relocate a GPU to a secure spot?
 I really want to avoid purchasing a new motherboard, but if I did I would prefer one where I could just swap my existing AMD Quadcore processor right onto it. From what I can tell, most boards with AM2+ sockets have the same PCI slot geography problem... they all have a PCIE located adjacent to a taller, probably obstructing old PCI.
Eureka. As it turns out, the PCIE extension cable ended up being the solution to the problem, but in an unexpected fashion. While plugging in the cable to the mobo PCIE x16 slot, I realized that the slot is somewhat tough to seat a card in on one side. Because of the bulk 6870 heat sink I could not easily see the slot while seating the card. With the circumstantial evidence that very few other people on the planet seem to have this problem (judging by the paucity of complaints on various hardware forums) and the fact that the motherboard geography seems to be common... I revised my diagnosis to: n00b mistake/operator error.
I unplugged the extension cable, gently realigned the 6870 with the slot and after a lot of patient manipulation, was able to successfully seat the card in the PCIE and expansion slots. It even worked when I turned it on ;-) now the only real estate problem I have is to replace a 9" SATA cable as it can no longer reach to the HDD around the girth of the 6870.
Thanks so much for your interest, sorry to waste your time ;-)