BleepingComputer.com: advice and couple Q's-new to building

I am going to do my first build but I have some questions. First can you guys look at my selection of parts and tell me if they will work together? (I'll be buying from newegg)
-Case: COOLER MASTER RC-690-KKN1-GP Black SECC/ ABS ATX Mid Tower link
-MB: GIGABYTE GA-MA790X-UD4P AM3/AM2+/AM2 AMD 790X ATX AMD link
-Video card: EVGA 512-P3-N871-AR GeForce 9800 GTX+ 512MB 256-bit GDDR3 PCI Express 2.0 x16 HDCP Ready SLI Supported link
-PSU: COOLER MASTER RS-700-AMBA-D3 700W ATX12V V2.3 SLI Certified CrossFire Ready 80 PLUS Certified Modular Active PFC link
-Heat Sink: ZALMAN CNPS10X Extreme 120mm 2 Ball CPU Cooler link
-CPU: AMD Phenom II X2 550 Black Edition Callisto 3.1GHz Socket AM3 80W Dual-Core Processor Model HDZ550WFGIBOX link
-RAM: A-DATA 4GB (2 x 2GB) 240-Pin DDR2 SDRAM DDR2 800 (PC2 6400) Dual Channel Kit Desktop Memory Model ADQVE1B16K link
-OS: Microsoft Windows Vista Home Premium SP1 64-bit for System Builders - OEM link

and my questions:
-I read somewhere it's better to buy PSU's of the same make as the case to ensure it will fit, but I doubt that (right?). Will this work?
-The two PSU's I'm comparing I noticed under "spec" there is "Modular." What does that mean? (So basically, why the price difference between the two?)
-I am most unsure about the heat sink. Do you think it will fit (in the case) and/or work with this configuration?

Also, I guess, if it all works then are there any cheaper components available with the same performance/quality that you would suggest?

Thanks

I'm no AMD "expert" but it looks OK to me.

"Modular means a p[ower supply has detachable leads so you only use the ones you need.
That has the advantage of less case clutter.
It has the disadvantage of some resistance at the connections causing slight power loss.
The power supply doesn't need to be the sams brand as the case unless you have a "brand name" unit that uses a proprietary power supply like a small form factor case.

Thanks for the reply.
I think I'll go with the StealthXStream PSU.

Also, is that heatsink&fan a good choice for $vs.quality? Are there suggestions for a more suitable/better heatsink&fan?

Everything looks good but there are few things you should consider. First, AM2/AM2+ motherboards and CPU's are old and are going to be phased out. The CPU you chose is AM3, but the motherboard isn't. Eventually, motherboard manufacturers will stop releasing BIOS updates for AM3 CPU's and you may find your future upgrade options limited. I suggest going for an AM3 motherboard and DDR3. If you want to go the AM3 route with that CPU, I suggest the ASRock M3A780GXH/128M AM3 AMD 780G HDMI ATX AMD Motherboard paired with G.SKILL 4GB (2 x 2GB) 240-Pin DDR3 SDRAM DDR3 1333 memory. Second, the CPU comes with a heatsink and fan in the box so technically you don't need one. The one you chose is good, but very large and some mid ATX tower cases cannot hold them. Next, I echo fairjoe's remarks about modular power supplies. They keep things more oraganized and the power loss from resistance is minimal. Finally, I suggest getting an ATI card since you're going for an AMD chipset. If you later want to use a second graphics card, you will be able to. The POWERCOLOR AX4870 512MD5 Radeon HD 4870 512MB offers a little better performance for the same price as a 9800GTX+. Some games can use more than 1GB of video card memory, if you want to spend the extra cash, the SAPPHIRE Vapor-X Radeon HD 4870 1GB would be good too.

I agree, AMD is my CPU of choice, and the upgrade capability is warranted. Enjoy your first build, when you turn it on, it will gratify!

Why go for a 9800GTX, when you can get a 4870 for $150 or less, that will destroy the GTX?