Let's see...
Newegg lists the ASUS M5A99X EVO for $149:99 and the 1055T for $149:99, which is roughly $300.
The i5 2500k is $219:99, which leaves around $80 for a motherboard. You won't find a good P67 or Z68 board for $80 unless you find a nice deal, but here are some examples at different price points (of course, landing you above $300):
MoBo: MSI P67S-C43 –
$99.99
MoBo: Gigabyte GA-P67A-D3 –
$104.99
MoBo: MSI P67A-G43 –
$114.99 Review
MoBo: ASRock Z68 Extreme3 –
$124.99
My vote's on the ASRock Z68 Extreme3, but either of the boards above would be just fine. If you aren't into overclocking, you might want to consider this:
i5 2400:
http://www.newegg.com/Product/Produc...82E16819115074
ASRock H67M (B3):
http://www.newegg.com/Product/Produc...82E16813157233
(Total: $270)
You can also shave some money off your planned build:
- The PSU is slight overkill; a good 450-500W is enough. For example:
http://www.newegg.com/Product/Produc...82E16817151094
- I'd upgrade the GPU to either a 6850 or a 6870. If you want to stick with nVidia, I'd probably recommend trying to upgrade the 550 Ti to a 560; the 550 Ti is a fairly weak gaming GPU. I believe the GTX 460 is a better choice as well.
- You don't have to pick up fancy RAM or an aftermarket heatsink (unless you plan to overclock).
Take a peak here for some further examples on builds:
http://www.mmo-champion.com/threads/...-Sample-Builds
As you can see, at $700 and $800+ a 2500k build is very possible. E.g.: