I already posted this on a tech support forum, but no answers yet, so:

I recently installed a GeForce GTS 250 in place of my old GeForce 9400 GT. The problem I'm having is that when I'm playing a game, my monitor will suddenly "turn off" and display the "No Signal" screen.

What's going on? I've tried the game at low settings and it runs fine, but the reason why I got the card is so I can run the game at max settings. Even at medium it goes out. Funny thing is, there's no lag in game, no drop in FPS at max settings. Even when my monitor's out, the sound's still working so my computer obviously had not crashed. The plug is tightly connected as well.

Here are my specs:

Pentium(R) Dual-Core CPU E5200 @ 2.50GHz
Memory: 4gb of ram
Hard Drive space: 500gb total, ~325 free space
OS: Windows Vista 64-bit
Video Card: Nvidia GeForce GTS 250

Also, the vid card only allows DVI connectors and my monitor's a VGA so I'm using a converter. I also just updated my driver for the card, if that helps.

Overheat? Check temperatures.

CPU temperature was at 33 - 36 Celsius
HDD teamperature was at 34- 36 Celsius

I highly doubt it's overheating

Your converter doesn't work very well or your monitor can't handle it.
That's all I can say for certain.

did the converter come with the gpu?

also, did you uninstall the old drivers from your previous graphics card?

What's your PSUs name/wattage?

Also, seems like either a massive case of overheating on the GPU (which I doubt), or something is being lost somewhere.

Make sure everything is connected properly (including your 6Pin PCIe connector) and make sure the card is lodged in there pretty firmly. Also invest some time into finding another converter you can try, seems like it could be the problem.

2-D, the converter came with the GPU and yes, I did uninstall the old drivers.

Alright, I know what's wrong now. GTS 250 needs a 450W power supply at minimum, and I only have a 400W power supply.

Anyone here knows what I should check before buying a power supply? My specs are already listed and as for my mobo, Foxconn G31MX-K.

Buy a Corsair power supply. It's a bit more expensive, but it's worth it.

Get this:

http://www.novatech.co.uk/novatech/prods/Components/PowerSupplies/Corsair/VX550W.html

Quote from STUFF

Anyone here knows what I should check before buying a power supply? My specs are already listed and as for my mobo, Foxconn G31MX-K.

make sure it's compatible with your mobo. most mobos take ATX. Also make sure theres the cord to plug it into your gpu.

Yeah, some guys over at the tech forum suggested corsairs too.

I'll be looking into that stuff, 2-D, thanks a bunch.