ThirtySixBelow is right. That stuff actually does increase the inside temps. It also doesn't really do an adequate job cancelling noise, but it does decrease the "whine" from fans.
The problem is...if you have a window in your case, the rubber noise mats won't do a thing for ya.
The best way to reduce noise is to use quieter fans, and a quieter PSU. I found that a good quality power supply, with good fans and fan speed settings does wonders for noise reduction.
CPU - P4 2.4C, 800FSB W/HT - CONSERVATIVELY OC\'ED
MOBO - GIGABYTE GA-8KNXP 2.0 875P CHIPSET/DUAL BIOS VER. FE
RAM - 1GB (2X512) OCZ PC3200 REV 2, 400MHZ 2-3-3 6
VID CARD - ASUS RADEON 9600XT 128MB W/VIVO
FLOPPY - SAMSUNG 1.44M
HDD - MAXTOR 120GB 7200RPM ATA133 EIDE W/8MB BUFFER
DVD - LG GDR8162B 16X IDE DVD ROM
PSU - ENERMAX EG465P-VE FC 460 WATT
VANTEC VP4-C7040 AEROFLOW HS/FAN
5 X VANTEC STEALTH 80MM CASE FANS