im wanting to upgrade my gateway fx400 to the x58 motherboard, i7 processor and geforce gtx 295 video card. will i be able to replace just those 3 things? will the computer's CASE allow the components to keep cool? im sure i will need to buy a more powerful power supply and some better type of cooling device. what else do i need to know? ive never done anything like this before. any suggestions on the power supply wattage i need? is 700 sufficient? what type of cooling DEVICES should i get? your help is GREATLY APPRECIATED. Why do you want to keep the case? Cut down on cost? The way your making it sound is like you have a computer in working condition and just want to upgrade to a better PC. The best way from a system builders STAND point and having experience with DELLs is to simply buy a new case. I have no idea about any other plug and play computers out there but i know dell case(atleast new ones) wont fit a aftermarket motherboard. Thomas901,
You have 3 threads open on this same issue.

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BTX compatibility with ATX products

The BTX form factor motherboards are incompatible with most of the ATX form factor cases and vice-versa. Moreover, cases such as the Cooler Master Series (Stackers) support a varying range of motherboard types such as ATX, BTX, Mini-ATX and so forth. However, all connectors are compatible, including power supplies, PCI cards, processors, RAM, hard drives, etc.

Criticism

BTX's thermal design specifies a particular processor location at the front of the board, where a special case ventilation duct is located. It places the northbridge behind the processor, and memory DIMMs beside the northbridge, arranged parallel to front-to-back airflow. Unfortunately, this is not possible for processors with an integrated memory controller (on-die northbridge), such as the Athlon 64 and Intel Core i7.[3] To equalize the length of the signal traces to the DIMMs, these processors must be centered in front of them. The DIMMs cannot go beside the processor, because they would then extend past the front of the motherboard, and they would obstruct the airflow if positioned behind it.