There are a lot of posts about MaxClients and HARD_SERVER_LIMIT in this forum or on the Plesk forum. RedHat ships the Apache RPMs with compiled MaxClients 256 and as most users have already noticed is futile to increase the number in httpd.conf.
4PSA compiled Apache 1.3.27-2 with a 2048 MaxClients support (as in Plesk standard) and made available the RedHat 7.3 RPMs at http://www.4psa.com/software/.
You can download the apache-1.3.27-2.i386.rpm and install it using:
rpm -Uvh apache-1.3.27-2.i386.rpm --force
These RPMs can be used with any control panel that require the Apache RPM.